House battery(ies) monitor
- Thread starter Ferg
- Start date
I can recommend a couple (but there are many good ones). First thing is that there are two different types.
One is a coulomb (amp) counter type. Of those I like the Victron BMV-700 or BMV-702 (702 can either monitor voltage for a second bank or keep track of temp). These are excellent for seeing what things draw (although of course once you know that once, it doesn't really change) and for keeping track of amps in and out. They give you a SOC (state of charge) in percent, but it can be subject to some inaccuracies. You can take steps to mitigate those (set bank size; drop size down as batteries age; disable or set certain parameters certain ways such as no auto 100% feature). This type requires a shunt to be placed in the negative line just as it comes off the house battery. Not that that's a big deal.
Type two works via algorithm. For example, Smart Gauge. This gives you a really accurate SOC (and voltage) but doesn't show you how many amps things use. It's basically a "SOC meter." Super easy installation (two small wires to battery posts) and no calibrating as battery bank size ages smaller.
I have one of each because I'm a nerd who likes data, and because their differences complement each other .
For pure SOC use, I prefer the Smart Gauge. For a great combo I like both. If someone only wants to buy one? Hmm, that's a tough one! Either is way better than nothing though, so you can't totally go wrong. I guess I'd have to take the Smart Gauge if I could only have one (but luckily there is no such rule
)
If you want to read more detail on them (with lots of photos) --- both for installation/setting up of Victron, and for Smart Gauge -- then Maine Sail has a few good articles at the following links. Of course there are also a great many knowledgeable people here - but in case you are interested in more reading:
Installing battery monitor (Victron amp counter type):
http://www.pbase.com/mainecruising/battery_monitor
Keeping above type more accurate:
http://www.pbase.com/mainecruising/prog ... ry_monitor
More about algorithm type (Smart Gauge):
http://www.pbase.com/mainecruising/smart_gauge
One is a coulomb (amp) counter type. Of those I like the Victron BMV-700 or BMV-702 (702 can either monitor voltage for a second bank or keep track of temp). These are excellent for seeing what things draw (although of course once you know that once, it doesn't really change) and for keeping track of amps in and out. They give you a SOC (state of charge) in percent, but it can be subject to some inaccuracies. You can take steps to mitigate those (set bank size; drop size down as batteries age; disable or set certain parameters certain ways such as no auto 100% feature). This type requires a shunt to be placed in the negative line just as it comes off the house battery. Not that that's a big deal.
Type two works via algorithm. For example, Smart Gauge. This gives you a really accurate SOC (and voltage) but doesn't show you how many amps things use. It's basically a "SOC meter." Super easy installation (two small wires to battery posts) and no calibrating as battery bank size ages smaller.
I have one of each because I'm a nerd who likes data, and because their differences complement each other .
For pure SOC use, I prefer the Smart Gauge. For a great combo I like both. If someone only wants to buy one? Hmm, that's a tough one! Either is way better than nothing though, so you can't totally go wrong. I guess I'd have to take the Smart Gauge if I could only have one (but luckily there is no such rule
)If you want to read more detail on them (with lots of photos) --- both for installation/setting up of Victron, and for Smart Gauge -- then Maine Sail has a few good articles at the following links. Of course there are also a great many knowledgeable people here - but in case you are interested in more reading:
Installing battery monitor (Victron amp counter type):
http://www.pbase.com/mainecruising/battery_monitor
Keeping above type more accurate:
http://www.pbase.com/mainecruising/prog ... ry_monitor
More about algorithm type (Smart Gauge):
http://www.pbase.com/mainecruising/smart_gauge
Specifically what batteries do you have now, and what are you replacing them with? What is the intended use of the batteries: For example, run refer 24/7 when cruising a month at a time, one day a week under way? Vs an occasion overnighter? How are you gong to be charing them? Shore, high capacity battery charger, outboard (output of our alternator).
How long did the old batteries last? Were they ever discharged more than 50%? (by less than 122 volt criteria at rest?
The options that Sunbeam suggested cover the ones often used. There are far cheaper versions, which have some negatives.
I do suggest that you might consider Group 31 batteries, vs group 27 which you probably have now. If you have higher loads, then there are even further options.
How long did the old batteries last? Were they ever discharged more than 50%? (by less than 122 volt criteria at rest?
The options that Sunbeam suggested cover the ones often used. There are far cheaper versions, which have some negatives.
I do suggest that you might consider Group 31 batteries, vs group 27 which you probably have now. If you have higher loads, then there are even further options.
Dr. Bob,
Don't know how long the prior batteries lasted, I bought her last year, and they were on their last legs. I'm guessing they were originals on my '08. Also they were dual starting/deep cycle.
I already bought two Group 27/90ah deep cycle, so so much for the larger ones.
We generally cruise, and really don't plan on using the heck out of the batteries. We will probably anchor out 3 days/summer month. I only need it to drive the microwave, tv, fridge, stereo and charge the Bluetooth speaker. With the exception of the fridge, there will be infrequent use of the others.
We keep the boat on a trailer at the house, so it's always smart-charged, plus we have a Honda 2000 on board. I just like to enjoy the quiet when I'm on the hook
Don't know how long the prior batteries lasted, I bought her last year, and they were on their last legs. I'm guessing they were originals on my '08. Also they were dual starting/deep cycle.
I already bought two Group 27/90ah deep cycle, so so much for the larger ones.
We generally cruise, and really don't plan on using the heck out of the batteries. We will probably anchor out 3 days/summer month. I only need it to drive the microwave, tv, fridge, stereo and charge the Bluetooth speaker. With the exception of the fridge, there will be infrequent use of the others.
We keep the boat on a trailer at the house, so it's always smart-charged, plus we have a Honda 2000 on board. I just like to enjoy the quiet when I'm on the hook
Good point on checking the type of batteries. For example, the Smart Gauge can't work with lithium batteries, because it relies on reading voltage. I assumed (not the best method!) that if a person were putting in lithium batteries they'd mention it, since they are still rather special. I figured flooded cell or AGM most likely (but should have asked).
Ferg: Your "only three days at anchor using microwave, fridge, TV, and stereo" would be maxing things out for me. That's a fair load (especially with the inverter/microwave which really hits the batteries) so I think you are smart to monitor SOC in some way. It doesn't take too many sets of batteries (less than one in some cases) to easily pay for a Victron (or similar) or SmartGauge. I had a friend a couple of years ago who went through many (!) sets of batteries at over $1000 a pop (set) before getting his Victron set up correctly and adding a SmartGauge. Of course it's not so dramatic if you are talking about a couple of $100 batteries, so maybe harder to justify. So of course there is no one right answer for everyone.
Ferg: Your "only three days at anchor using microwave, fridge, TV, and stereo" would be maxing things out for me. That's a fair load (especially with the inverter/microwave which really hits the batteries) so I think you are smart to monitor SOC in some way. It doesn't take too many sets of batteries (less than one in some cases) to easily pay for a Victron (or similar) or SmartGauge. I had a friend a couple of years ago who went through many (!) sets of batteries at over $1000 a pop (set) before getting his Victron set up correctly and adding a SmartGauge. Of course it's not so dramatic if you are talking about a couple of $100 batteries, so maybe harder to justify. So of course there is no one right answer for everyone.
In small boats where I have had a microwave I have dedicated batteries to the inverter, or very robust batteries. For example, I probably would want over 90 amp hours dedicated to the microwave. (depending on use). The inverter will probably draw 100 amps when running the microwave.
You have 90 amp hours "usable". Depending on the ambient temp, you are going to be pushing the reserve with the refrigerator alone, if you don't charge batteries every day. I probably would add a 3rd 90 amp hr group 27 to your bank, especially in a 2008 25, where there is plenty of room for the extra battery. Use the two group 27 in parallel as the "house bank" and monitor these. Keep the other group 27 as a starting battery. Add a VSR to charge the "house bank" or if your motor has a second charing circuit use that for the "house bank".
Then using the Victron or similar monitor (I use a Link--same idea), you will have a good handle on when you need to charge the batteries.
Unfortunately the battery charger which comes with the boats is fine for topping off the bank at home or at the dock, but not for charging the batteries from the Honda EU 2000. I carry a 30 amp marine charger just for the refer battery, and then use my upgraded Mastervolt battery charger for the other batteries.
You have 90 amp hours "usable". Depending on the ambient temp, you are going to be pushing the reserve with the refrigerator alone, if you don't charge batteries every day. I probably would add a 3rd 90 amp hr group 27 to your bank, especially in a 2008 25, where there is plenty of room for the extra battery. Use the two group 27 in parallel as the "house bank" and monitor these. Keep the other group 27 as a starting battery. Add a VSR to charge the "house bank" or if your motor has a second charing circuit use that for the "house bank".
Then using the Victron or similar monitor (I use a Link--same idea), you will have a good handle on when you need to charge the batteries.
Unfortunately the battery charger which comes with the boats is fine for topping off the bank at home or at the dock, but not for charging the batteries from the Honda EU 2000. I carry a 30 amp marine charger just for the refer battery, and then use my upgraded Mastervolt battery charger for the other batteries.
Actually I do have two paralleled 90ah hours hoise batteries along with a separate starting battery.
I'm not sure if I have the original charger; it currently has a Guest 20 amp duel (5,5,10).
Dr. Bob, can you why the Honda 2000 won't be sufficient to charge. This is out of my area of expertise (that makes it sound like I actually have an area of expertise)
I'm not sure if I have the original charger; it currently has a Guest 20 amp duel (5,5,10).
Dr. Bob, can you why the Honda 2000 won't be sufficient to charge. This is out of my area of expertise (that makes it sound like I actually have an area of expertise)
journey on
New member
Well, Dr. Bob didn't say that the Honda 2000 wouldn't charge your batteries. He did say that your battery charger isn't adequate.
The Honda just supplies 110 vac, up to 1800 watts which is more that adequate for the job. The battery charger turns it into the required DC and supplies it to the batteries.
There was a thread about the lack of performance of the small OEM charger and with what we replaced it.
Boris
The Honda just supplies 110 vac, up to 1800 watts which is more that adequate for the job. The battery charger turns it into the required DC and supplies it to the batteries.
There was a thread about the lack of performance of the small OEM charger and with what we replaced it.
Boris
The Honda EU 2000 only puts out about 8 amps with its 12 volt battery charging circuit. The "factory charger" which you have will not charge the batteries rapidly. It will be limited to 5 amps to each of the two "house" batteries.
I have a separate 30 amp charger (runs off a Honda EU 1000) which I run for a little less than 2 hours to bring two Group 31 batteries up after running two chest refer/freezers. You can certainly put in 20 amps--and probably 30 amps into the two 90 amp batteries you have. Good to know that you have the two battery house bank! With the monitor you can watch the voltage and SOC.. If you wanted to use the 12 volt circuit from the Honda, it would take about 12 to 15 hours to come up to full state of charge. Charge tapers off as it reaches the "float stage" The last 10% of charge takes a long time,
I have a separate 30 amp charger (runs off a Honda EU 1000) which I run for a little less than 2 hours to bring two Group 31 batteries up after running two chest refer/freezers. You can certainly put in 20 amps--and probably 30 amps into the two 90 amp batteries you have. Good to know that you have the two battery house bank! With the monitor you can watch the voltage and SOC.. If you wanted to use the 12 volt circuit from the Honda, it would take about 12 to 15 hours to come up to full state of charge. Charge tapers off as it reaches the "float stage" The last 10% of charge takes a long time,
My engine alternator (Honda BF90) will put a lot more amps into the battery than my Honda 1000 will through the battery charger. The generator charging is limited by the battery charger current (in my case 5 amps/channel).
However, even when the battery charging is not limited by the charger, I think that engine powered battery charging is more efficient. A Honda 2000 will only give you about 18 amps to a single battery if your charger is single channel and the current is not limited by the charger. A dual channel charger will only give you half that, and AFAIK dual channel chargers do not combine currents from the other channel if one battery is already charged.
However, even when the battery charging is not limited by the charger, I think that engine powered battery charging is more efficient. A Honda 2000 will only give you about 18 amps to a single battery if your charger is single channel and the current is not limited by the charger. A dual channel charger will only give you half that, and AFAIK dual channel chargers do not combine currents from the other channel if one battery is already charged.
ssobol":2n1zq6s6 said:
The amount of charging by the Honda 90 is dependent of year--2006 and prior are only 17 amps output--and part of that is used by the engine. Those after 2007 35 amps.
I don't understand why you say that a Honda EU 2000 will only give you 18 amps to a single battery? A Honda EU 1000 will easily ;power a 30 amp battery charger, plus. Various types of batteries have limited amount of current inflow--depending on capacity and type of battery, but that is not addressed in your post.
For example in the RV, I routinely run a Magnum inverter charger 110 amp 12 volt charger into 440 amp hour battery bank which draws 14 amps @110 amps output--within the range of an EU2000~! Please explain.
Good quality dual and tri channel chargers will put out full current to each channel if necessary. For example the Mastervolt, 40 amp 3 channel charger will deliver 40 amps to one channel or battery, if necessary, and programed that way. It will deliver what is necessary up to 40 amps total.
This battery charger is easily run with a EU1000, and two could be run by an EU 2000. Both the EU 1000 and 2000 will only put out 8 amps at 12volts DC output stage, but that does not in any way limit the amount of 110 volt charging capacity available.
By my calculations a Honda 2000 at 120v gives you 15 amps output (at the full 2000 watts). With the right charger you could get about 40 amps output to the battery with this input. In this example, a Honda 1000 might give you 20 amps charging current.
With my Guest 20 amp 3 bank charger, the input current at 120v is 5a max. The output is 5/5/10a at 12V. This charger does not automatically combine outputs to supply max available current to a single channel. You can combine outputs via the wiring, but this is not dynamic.
Apparently in my setup I can get 5 amps max into one battery and 15a max into the other one (two channels in parallel).
My BF90D will put about 28.5 amps into the battery at anything >1000 rpm. At idle it is ~17 amps (for a battery at 70% charge) according to the Victron battery monitor.
It would seem that the battery charging on my boat is limited by the charger more than the available input energy.
With my Guest 20 amp 3 bank charger, the input current at 120v is 5a max. The output is 5/5/10a at 12V. This charger does not automatically combine outputs to supply max available current to a single channel. You can combine outputs via the wiring, but this is not dynamic.
Apparently in my setup I can get 5 amps max into one battery and 15a max into the other one (two channels in parallel).
My BF90D will put about 28.5 amps into the battery at anything >1000 rpm. At idle it is ~17 amps (for a battery at 70% charge) according to the Victron battery monitor.
It would seem that the battery charging on my boat is limited by the charger more than the available input energy.
As per Honda EU 2000i Specs:
The Honda EU 1000i specs:
I have no idea where you are getting the "charger output of 40 amps" from a EU 2000I, or 20 amps from a EU 1000i. But I run a West Marine branded, portable battery charger, output (measured with external amp meter) of slightly over 30 amps, plus other light loads. (Charging 2 AGM group 31 110 amp hour batteries) The EU 1000 will not run the Mastervolt 20 amp, plus the Torqeedo charger, plus the 30 amp output. However by the time the 30 amp charger is down to about 20 amps, the EU 1000 will run all 3. (which would be about 7.5 amps at 110 volts.)
My documented experience says that your "calculations" are not correct. Yes, some battery chargers are more efficient than others.
Lets take this to the theoretical level: The most efficient battery chargers will put out 85% of the input amperage: Assume that the Honda is putting out 120 volts, and the battery charger is putting out 12 volts (reality is that the battery charger is putting out 14.3 volts at 30 amps). This means that there is a factor of 10:1, or 7.5 amps of 120 V = 75 amps at 12 V : 85% of that would be 63.75 amps or the theoretical 12 volt out put of max current production of the EU1000..
Since the true battery voltage is 14.3 at output, solving the equation shows that theoretical max output at 14.3 volts would be 53.5 amps of 12 volt DC. Not 20 as stated.
Unfortunately the Guest chargers supplied with our boats are not the most efficient chargers. My experience is that they only last a few years, especially in regular use. This is the reason I have had to replace every Guest charger that has come with my C Dorys. This is often the case with production boats. The custom and more expensive boats often come with a higher quality charger--such as Victron, Blue Seas, VMI (Macaroon) or MasterVolt. These are far more expensive, and generally are far more reliable as well as being more efficient. They are also programable for different type of batteries, and some being able to independently charge each battery to its best perimeters.
d The "rated" is continuous output.
The Honda EU 1000i specs:
I have no idea where you are getting the "charger output of 40 amps" from a EU 2000I, or 20 amps from a EU 1000i. But I run a West Marine branded, portable battery charger, output (measured with external amp meter) of slightly over 30 amps, plus other light loads. (Charging 2 AGM group 31 110 amp hour batteries) The EU 1000 will not run the Mastervolt 20 amp, plus the Torqeedo charger, plus the 30 amp output. However by the time the 30 amp charger is down to about 20 amps, the EU 1000 will run all 3. (which would be about 7.5 amps at 110 volts.)
My documented experience says that your "calculations" are not correct. Yes, some battery chargers are more efficient than others.
Lets take this to the theoretical level: The most efficient battery chargers will put out 85% of the input amperage: Assume that the Honda is putting out 120 volts, and the battery charger is putting out 12 volts (reality is that the battery charger is putting out 14.3 volts at 30 amps). This means that there is a factor of 10:1, or 7.5 amps of 120 V = 75 amps at 12 V : 85% of that would be 63.75 amps or the theoretical 12 volt out put of max current production of the EU1000..
Since the true battery voltage is 14.3 at output, solving the equation shows that theoretical max output at 14.3 volts would be 53.5 amps of 12 volt DC. Not 20 as stated.
Unfortunately the Guest chargers supplied with our boats are not the most efficient chargers. My experience is that they only last a few years, especially in regular use. This is the reason I have had to replace every Guest charger that has come with my C Dorys. This is often the case with production boats. The custom and more expensive boats often come with a higher quality charger--such as Victron, Blue Seas, VMI (Macaroon) or MasterVolt. These are far more expensive, and generally are far more reliable as well as being more efficient. They are also programable for different type of batteries, and some being able to independently charge each battery to its best perimeters.
journey on
New member
Hello!!!!
Will everyone start labeling their outputs as charger or generator? And use the terms AC or DC?
Bob's correct on what the Honda generators are putting out and that's AC
One can get 40 amp DC @ 13.5 volts DC out of a battery charger (13.5x40=540 watts DC) if one puts 4.5 amps AC at 120 volts ACx2 (960 watts AC) in from the generator. The factor of 2 is used because the charger is less than 50% efficient.
What's not mentioned is that with flooded lead-acid batteries the charge rate goes down rapidly once charge is started. That 40 amps DC will soon drop quite significantly to 20 A DC, then to 10 amp DC, etc.
And finally, here's a discussion on the Guest Charger.
Boris
Will everyone start labeling their outputs as charger or generator? And use the terms AC or DC?
Bob's correct on what the Honda generators are putting out and that's AC
One can get 40 amp DC @ 13.5 volts DC out of a battery charger (13.5x40=540 watts DC) if one puts 4.5 amps AC at 120 volts ACx2 (960 watts AC) in from the generator. The factor of 2 is used because the charger is less than 50% efficient.
What's not mentioned is that with flooded lead-acid batteries the charge rate goes down rapidly once charge is started. That 40 amps DC will soon drop quite significantly to 20 A DC, then to 10 amp DC, etc.
And finally, here's a discussion on the Guest Charger.
Boris
thataway":nyy7rz0s said:
According to the specs for a Mastercraft charger I found online, the Max input current for that charger was 120vac at 16A. This gave a max charging current of 12VDC/40A. Assuming that the Honda 1000 only has half the current output of a Honda 2000 (more or less) I estimated that the 1000 can give a max charging current of 20A at 12VDC.
Different chargers/inverters might be more or less efficient. I figure that the one I used was a reasonable example.
ssobol":2vn65azo said:
Sorry, I cannot find any "Mastercraft" charger on line. I have experience over several years, running a 30 amp charger plus off a EU 1000I, as stated above.
I have currently a Mastervolt power charger 12/20-3 which will run fine along with my secondary 30 amp charger, when the 30 amp charger is down to 20 amps. (amp meter measured). When I get home, I can measure accurately the amps 120 V AC input of all of my battery chargers when I get home.
I regularly use the "Kil-o-watt" meter to measure the 120 V AC amps/watts draw of any "appliance" I put on the boat. I also use one of several analogue or digital volt/amp meters to measure the 12 V DC output current.
In the past I had another 4 years experience running a 40 amp Xantrex True charge, plus the standard guest 10/5/5 on the Tom Cat, on a Honda EU 2000, with no issues.
I have over 7 years experience running a 110 amp 12 V DC charger where I measure the direct 120 Volt AC amperage into that charger (part of the read out of the system).
smckean (Tosca)
New member
I can follow the discussion (altho there is some talking "past" each other); however, this last stmt by you does confuse me. I'm just checking in to be sure I am not missing something.ssobol":3fm8yzq2 said:
If I start with physics (always a good place to start :wink: :lol: ), energy is conserved....energy-in must equal energy-out. In this situation, seems to me the useful measure of instantaneous energy would be watts (that is, the amount of energy flowing at any single moment). Ignoring losses for now, the watts of the generator must equal the watts of the charger. Since watts = volts x amps, and assuming we are talking 120v AC and 12v DC, the max amps available to charge the battery via a "perfect" charger will be about 10 times the max amps the generator can deliver.
So now my confusion: how can 16 amps of 120v AC at the charger only make available 40 amps of 12v DC. It seems it should be 160 amps of 12v DC. OK...there ARE losses due to inefficiencies etc. Someone mentioned a 50% figure (which seems reasonable to me), but ssobol, your numbers imply that only 25% of the energy expended by the generator actually ends up in the battery. Is this process really that inefficient??
The limitation seems to be in the battery charger. Some of it is efficiency, but a lot of it is design choices by the charger manufacturer. Yes if you do the math, 2000 watts at 12 volts will give a lot of current. But a charger that can supply 166 amps to a battery would be larger, heavier, and more expensive than one that only charges at 10 amps. Further, for the typical group 24 or 27 battery used in our boats it might be unwise to charge at that rate anyway.
smckean (Tosca) Basically you are correct--but as I pointed out in the minor math equations I did, the very best charger is only about 85% efficient. Then you have to realize that the charger is putting out 14.3 volts at its max bulk phase, so that further reduces the amount of current available, for charging. The older chargers were far less efficient, with large transformers and rectifiers. Now the chargers are almost fully solid state, and diodes/transistor.
The cheaper battery chargers are...cheaper to build... The Guest chargers which are in the boat, are made to top off batteries (nominally the 20% which is not topped off by running the boats' engine, and then maintaining the battery, or doing a slow charge. They are expected to do their work over many hours. One of the reasons they fail, is that they are pushed to their max capacity when trying to charge depleted batteries after overnight, with a refrigerator, stereo, GPS, depth finder etc, running, and battery discharged to over 50% capacity. The Guest chargers are just not made for this type of use. The bigger and more expensive chargers have fans, temp sensors, and are programed to charge batteries based on several factors.
Generally no more than 25% of the capacity is the charge rate recommended for a flooded lead acid battery. AGM can accept more charging capacity without harm. In either case, there should be temperature monitoring and compensation for high charging rates. For example the 110 amp battery charger, is charging a 440 amp hour battery bank. There is a temperature sensor attached to the positive pole of the middle of this bank. This keeps the charger from over charging the battery (heat is bad for battery life).
The 30 amps I use is going into a bank of two AGM group 31 batteries, (total 260 amp hours) and also has temperature compensation built into the equation. I certainly would not charge the usual battery found on a C Dory with a group 24 battery (50 to 75 amp hours), or medium capacity group 27 battery (70 to 100 amp hours) at 30 amps. My two group 31` AGM batteries are dedicated to running a chest type freezer and refrigerator. Thus a rapid charge time helps keep generator run time down.
The cheaper battery chargers are...cheaper to build... The Guest chargers which are in the boat, are made to top off batteries (nominally the 20% which is not topped off by running the boats' engine, and then maintaining the battery, or doing a slow charge. They are expected to do their work over many hours. One of the reasons they fail, is that they are pushed to their max capacity when trying to charge depleted batteries after overnight, with a refrigerator, stereo, GPS, depth finder etc, running, and battery discharged to over 50% capacity. The Guest chargers are just not made for this type of use. The bigger and more expensive chargers have fans, temp sensors, and are programed to charge batteries based on several factors.
Generally no more than 25% of the capacity is the charge rate recommended for a flooded lead acid battery. AGM can accept more charging capacity without harm. In either case, there should be temperature monitoring and compensation for high charging rates. For example the 110 amp battery charger, is charging a 440 amp hour battery bank. There is a temperature sensor attached to the positive pole of the middle of this bank. This keeps the charger from over charging the battery (heat is bad for battery life).
The 30 amps I use is going into a bank of two AGM group 31 batteries, (total 260 amp hours) and also has temperature compensation built into the equation. I certainly would not charge the usual battery found on a C Dory with a group 24 battery (50 to 75 amp hours), or medium capacity group 27 battery (70 to 100 amp hours) at 30 amps. My two group 31` AGM batteries are dedicated to running a chest type freezer and refrigerator. Thus a rapid charge time helps keep generator run time down.