Can I charge/maintain 12V batteries in parallel?

ShoelaceExpress wrote:

So while connected in parallel in my garage, I'd have the batteries connected positive to positive, and negative to negative, and then connect my charger/maintainer to the negative on one battery and positive on the other battery?

When connected in parallel on my trailer, just connect the charge controller in the same way?

When on trailer, you should interconnect batteries Pos to Pos, Neg to Neg, and then connect trailer cables to Pos and Neg on different batteries. Not on the same battery. Controller should be wired in the same manner. This is called a balanced wiring. Less important during maintenance charging in garage.

As said, many controllers don't like being connected to solar without being connected to battery first. Read the manual - a better model will probably mention this, cheap Chinese thingies might not, but they don't usually put much thought in manual writing - so I would still follow same procedure.

Agreed with Lwiddis - 2*40W is a very small solar. More than enough for storage, hardly enough for living. With only fridge, pumps and few lights it might keep up on a sunny day. $CAD 180 is a good price for Canada. Though, knowing Canadian Tire, you controller is very, very likely bottom of the barrel, keep an eye on it. And on those SAE connectors too - not the best type for outdoors.

RoyB said, "may be a challenge for me due to available space on the roof and the weight of the panels involved."

Space might be an issue but weight shouldn't. Four 100 watt panels should give you 20 amps for less than 80 pounds.

"For while camping, I bought 2x40W Coleman solar panels (80W total.)"

Two forty watt solar panels will keep your batteries charged during storage but they won't be sufficient to recharge your batteries when camping during normal use. I'd try to return them and restart research regarding solar systems. If your two batteries are group 24s with 90 amps or so each you'll need 200 watts of solar minimum.

ShoelaceExpress wrote:

As for the converter/charger in the trailer, I'm not sure what DC output current it's rated for. I will have to check next time I'm there. Where would I look? Likely behind a panel near the circuit breaker panel?

Just fine the model and part number from the front of the converter/charger or inside the large door opening...

Then you can search for that model number on-line and get that off of the specs sheet. You should have a manual for the unit as well in your trailer somewhere...

The trailer on-board converter/charger will take better care of your batteries automatically if it is a smart mode charger. All you will need to supply is the 120VAC from the Shore power connection or a generator. The solar panels will only IMO be a trickle charge for your batteries when in high sun... Since the two panels can't produce the 17-20AMPS then you can't rely on battery science to charge your two batteries in a three hours run time. If the sun is high all day long those panels will get the job done before you lose the high sun in a days time. Just be aware the SUN isn't always out and some days it will be cloudy, or raining, or you are parked under a shade tree. Got to have the battery charging routine established around a generator in my book and then take advantage of the the high sun when it is there. In order for the battery science to work for you is when you initially hit the batteries with the 14.4VDC Boost charge voltages you will see a big increase of around 40AMPS DC current coming from the converter/charger and in an hours time this will taper back to around 8AMPS DC current as the batteries start taking on the charge. This is what happens with my PD9260C converter/charger at any rate. Then you shut down the generator and let the high sun continue on charging your batteries until the sun goes down that day. The solar controller box will do the same functions as your smart mode converter/charger does charging your batteries. Those two panels may only have 4 Amps DC current capacity or slight more coming from each panel in the high sun... Most likely enough to get the batteries charged in their tapered back state before the high sun goes down charged. It may even get them charged going all day long without the generator running the initial high DC current period required.

You have to get all of this stored into the battery bank before the sun goes away so you can get through the night with no high sun running off the batteries...

I know what would happen in my case being out off grid somewhere and not have my generator with me. The sun would not be high in the sky all the time haha... Mother Nature loves to play those tricks on you... You will look around and finally see a bird in the tree laughing his butt off at you for not having the generator going for the first hour of the charge period hehe...

Got to have the good PLAN Bs to be successful about all of this...

Getting solar on my off-road POPUP trailer may be a challenge for me due to available space on the roof and the weight of the panels involved. I will need at least two 100WATT size panels on the front end and a 240WATT panel across the rear end to get the the min 20AMPS of DC current capacity... It may be that I will have to get a folding dual 100WATT panel and only mount one large panel on my popup roof. First all I want to do is get my updated battery bank going then I will start looking into using solar panels to supplement my battery charging.

I have been playing with just batteries and a 2KE generator for charging since 2009 in my off-road POPUP trailer... My PD9260C converter/charger only needs 1000WATTS on the 120VAC side to operate it. I can run my 2KW generator in ECO mode and it revs up real high at first and then starts running slower as the batteries start taking on their charge. I can almost tell when it would be ready to shut down the generator just by listening to it and let solar panels take over.

Kinda exciting now that I am starting to think about some solar panels now...

Kinda neat to setup back in the woods somewhere with no hookups and have extension cords running away from the trailer like you see here in this camping photo running some 120VAC fans or whatevers with your camp setup... Folks come wandering by asking how do you do that haha... I always answer magic beans or dilithium crystals. Running fans at the camp site table area keeps the mosquitoes away the best hehe...

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Roy Ken

Thanks westend. I'll look at the back of the panels to see if any modifications are needed for wiring, and look into if the controller needs to be mounted first and possibly leave it hard wired as you suggest.

Good idea on the hinges as well on the panels.

ShoelaceExpress wrote:
I paid $180 CDN + tax (on sale) for the two 40W panels. I hadn't done much research before now. I liked the idea of having the two 40W panels as they're about 14" wide and when traveling I can put them face to face (maybe with a soft cloth between) and strap them together - easy to transport around. I want to keep them portable instead of mounting, or else I would have looked into larger options for roof mounting.

I hadn't thought of the solar panel idea of maintaining the batteries while in storage. With them sitting in my garage I would have to figure out a way to mount the panels outside. I don't mind keeping the trickle charger connected inside the garage though.

Thanks for the link. I may also look for a similar connector to leave permanently connected to the battery and leave one end of the SAE connector (capped) dangling outside the battery box so I don't have to unstrap the boxes every time I want to charge the batteries with the solar charge controller.

Good plan with the connections but you may find that the controller needs to be connected first so that can be hard wired and the panel connection made to the controller. Some controllers will fail if not connected to batteries first.

If the panels have a covered terminal box on the back, you can remove the cover and put any manner of wire or connector on the terminals.

For portability, I'd suggest to use two stainless butt hinges and fasten them to the panel sides if you have an aluminum frame. The panels can then be hinged together with faces together for transport.

Wow, thanks for the great write up Roy.

The two batteries are MotoMaster Nautilus Group 24 Starting & Deep Cycle Marine Batteries. 675 marine cranking amps, 65 amp hours, 120 minutes reserve capacity. I realize now these batteries aren't all that great after reading some other threads here talking about AGM and in-series 6V golf cart batteries, but I'm stuck with what I have now since they can't be returned.

The charge controller for the solar is a Coleman 7A.

The charger/maintainer I have in the garage is a MotoMaster Eliminator Intelligent Battery Charger, 12/8/2A.

As for the converter/charger in the trailer, I'm not sure what DC output current it's rated for. I will have to check next time I'm there. Where would I look? Likely behind a panel near the circuit breaker panel?

ShoelaceExpress wrote:

The new TT needed a new battery, so I ended up purchasing two deep-cycle RV/marine batteries to install in parallel.

It's too late now but two 6 V in series is really a better arrangement.

Nothing seriously wrong with what you propose to do and for all intents and purposes the two together can be treated just as one larger battery.

Yes, a disconnect switch would be good. Or just disconnect the negative battery cable.

The only potential problem with a parallel arrangement is: If one battery goes bad or starts getting weak, it will drag the other one down with it.

I suggest that each year when you take them into the garage, you charge each one individually first and check the specific gravity and resting voltage after removing the charger for 24 hours. IF they still seem to match, connect them in parallel again and attach the battery maintainer. It needs to have a capacity of AT LEAST 2 amps and must NOT be just a cheap trickle charger.

I paid $180 CDN + tax (on sale) for the two 40W panels. I hadn't done much research before now. I liked the idea of having the two 40W panels as they're about 14" wide and when traveling I can put them face to face (maybe with a soft cloth between) and strap them together - easy to transport around. I want to keep them portable instead of mounting, or else I would have looked into larger options for roof mounting.

I hadn't thought of the solar panel idea of maintaining the batteries while in storage. With them sitting in my garage I would have to figure out a way to mount the panels outside. I don't mind keeping the trickle charger connected inside the garage though.

Thanks for the link. I may also look for a similar connector to leave permanently connected to the battery and leave one end of the SAE connector (capped) dangling outside the battery box so I don't have to unstrap the boxes every time I want to charge the batteries with the solar charge controller.

I have found with my parallel wired 12V battery bank the multiple batteries in parallel will charge in the same amount of time as a single battery if all batteries are wired properly using the same lengths of cabling paths but you have to have a smart mode converter/charger unit that can produce 17-20AMPS of DC current per battery.

In the case of two batteries the size of the converter/charger unit should be a minimum of 45AMP DC output. Most of the single battery trailers usually have a 55 DC Output rated converter/charger.

Please post the brand and model of the converter/charger unit and the folks on here can tell you if that needs replaced if you want to re-charge the batteries from a 50% charge state to a 90% charge in around a three hour time frame. To get them to a 100% charge rate it will normally take around 12 hours of charge rate if the converter/charger is rated at 45AMPS or 55Amps. The camping off-grid idea is to start out with a 100% charge state battery bank and when the batteries drop to around the 50% charge state you will want to charge them back up to at least the 90% charge state which will take around three hours time. Then you can get almost full performance out of the batteries the next day/night run off the the two batteries again. You can do a good 12-14 of these 50% to 90% charge state before you must do a full 100% charge state. This of course will take a good 12hours of charging to get to a good 100% charge state. This is all battery science when using the stated standard DC VOLTAGEs that are picked by the battery folks designed to not boil out your battery fluids over the best timing they can do getting to the full 100% charge state.

If your converter/charger is only one of those smaller 30A DC OUTPUT units then it will take much longer to recharge the batteries.

Another issue might be to increase the wiring size from the converter/charger if it is small to a larger AWG size. I would suggest to use a 4AWG size wiring cable going from the converter/charger unit to the battery terminals.

This is also a great time to change out your internal lights from the incandescent type to LED type. This is a hugh savings in power consumption and getting the same amount of light. A typical incandescent automotive type light bulb will draw around 1AMP of DC current. The LED replacement unit for this light will only draw around .2 AMPS DC current. When you have some 12-14 of these lamp bulbs in the ceiling of your trailer this adds up big time...

Just food for thought - Confirm all of this with battery experts on here... I am just a self trained battery user with no formal battery training expertise and all of this is based on my personal experience using my 12V battery bank in my off-road camping conditions...

I have been using my three standard issue 85AH 12V Interstate batteries wired in parallel since 2009 and two of these batteries has finally got to the point of hold their good performance just last season.


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These three batteries are wired in this fashion. Was originally four GP24 85AH batteries but I lost one due to over charging in the first month and the battery fluids boiled out.


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My new battery bank in the planning stage now will look something like this drawing..

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The new batteries will be mounted across the same trailer tongue position but will be using one of those Torklift battery boxes with slide off metal top.

Roy Ken