Deep cycle batteries connected in parallel 2

[Tunalover]

Guys-
I'm a mechanical engineer. My electrical engineering knowledge stops at Physics III and Circuits I so I'd appreciate it if you could keep it simple. What I'd like to know about is the feasibility/desireability of connecting two (or more) 12VDC deep cycle batteries in parallel in an effort to boost the total amp-hrs available to an electric trolling motor. I have vague memories of someone saying that this is not good for one or both of the batteries. Is this true and, if so, is there a simple circuit that will protect the batteries but still come close to doubling the number of amp-hrs that would otherwise be provided by a single battery?


Tunalover

 

[felixc]

The best choice will be to find any single battery that can handle the A-h that you need. This way the standard battery charging methods do the job. Otherwise, everything gets more complicated, as you will need to monitor each cell independently for a real control, unless you're ready to change batteries on a regular schedule.

There is a "battery engineering forum" where some more people can give opinions.

 

[IRstuff]

The issue is that the battery with the higher voltage will try to charge the one with the lower voltage and wind up discharging a big chunk of its capacity for naught.

TTFN

 

[sreid]

Paralleling marine batteries is apparently commonly done. I think what makes it OK is if you use the same size batteries and they are in good condition (no cells shorted). Some will disagree about this but search the web.

 

[IRstuff]

Sure, it's done, but just because everyone's speeding doesn't make it right or correct.

You get what you pay for. If you ignore good engineering practice, when the sh*t hits the fan, the failure will be spectacular and expensive.

TTFN

 

[itsmoked]

tunalover:
For your application there is no reason you can't parallel two batteries. Putting batteries in series is a lot bigger problem. All most all "cruising" sail boats put many batteries in parallel and cycle them heavily for years
with few problems. I would not hesitate to do it. If you don't run them way down all the time you will get better life out of them to boot.

I have done this many, Many, MANY times.

Always recharge them as soon as possible.

I would recommend that they be the same brand and preferably the same model. But if not then they MUST be the same chemistry! Gel or flooded, etc.

 

[Paddymagee]

If you must go this way then be sure to
(1) Replace both batteries together even if only one is "dead"
(2) chaeck that your alternator / charger can cope with double the load when charging.

 

[VEBill]

Putting things in perspective - wiring cells in series is probably (in theory) worse than putting batteries in parallel. If one cell is weaker than the others then it can be driven into reverse voltage during the deep discharge cycle. Now consider that every typical '12-volt' battery is six cells in series ('The End Of The World Is Nigh' NOT!).

Paralleling a couple of batteries to increase A-hr is very common and there are few significant issues.

Chargers and alternators should self-limit their current to their own capability - otherwise they'd blow up the first time they met a dead battery. This is not an issue. The charging current doesn't automatically double for two batteries in parallel, but the charging time may double.

If one battery dies early (for some reason), then there may be no reason to replace the other battery. If it dies of old age, then perhaps you'd want to replace both at the same time.

If the batteries are kept in parallel then they'll obviously have the same voltage. If one does discharge into the other, there will be some loss. But once they're equalized, then this isn't an issue any longer.

Don't parallel connect a fully charged battery across a dead battery because the charging current between the two batteries might be incredible.

Watch out for hydrogen build-up.

Don't accidently connect one battary backwards (putting them in series). Make sure you have lots of light in the battery compartment so you can see.

 

[OperaHouse]

You'll get better advice from an alternate enetgy site like

http://www.otherpower.com

The general rule is to use the same type of battery and the same age within 6 months. In real life it doesn't matter in your aplication. You will only get a couple years of life out of the battery because of it being stored for long periods of time without use. Those "buy a bigger battery guys" haven't carried around too many deep cycle batteries!

 

[itsmoked]

Yeah heavy..hehehe. I had to get a 600lb battery outta a boat, onto the dock, up the ramp, into my truck. It wasn't a lot of fun...

On another note, sort of touched on by VE1BLL, two batteries in parallel can drive you nuts when one dies, as they both may read the same voltage but one may have zero capacity. So make sure you have a way to isolate them for testing. And test them individually at the first hint of any problem or you will be murdering the good one by radically discharging and recharging it, since the other one is there in *looks* only.

I have a diesel with two batteries in it, in parallel, and have learned this little tid bit first hand.

 

[rmw]

Everybody take a deep breath here.

Batteries, and even deep cycle batteries, when they are called for are commonly mounted in parallel, 2, 3, and 4 at a time on over-the-road transportation equipment such as trucks and busses. Why even my diesel pick up truck has two batteries in parallel.

They do their job, and alternators recharge them day in and day out, often without knowing how many batteries are in the battery compartment. They get changed out individually, and as complete sets. Not a big deal. (I prefer changing out by sets, but have done it individually with success.)

While a lot of theory has been thrown out here, and some good caveats given, practically speaking, there is no problem, tunalover, with your paralleling 2 12V batteries in order to get more fishing time in with your trolling motor. Enjoy yourself, and think about fishing while you are out there, not engineering.

rmw

 

[ScottI2R]

Tunalover,
I have for years paralleled a group 31 and a group 27 12V Deep Cycle battery on my boat. I do not usually run more than one at a time except during "charging." I have a make b4 break switch (MFG'd by PERKO) that allows switching during engine on operation. Although I do not have one,"battery isolators" are also available to prevent one from killing the other. (At least that is what I think they are for, not sure though....never needed one).
See, I use mine for obscene stereo operation. READ: Providing tunes for everyone else out at the island or sandbar on our local lake. I do not know what your trolling motor draws, but my twin amps draw over 30A each. This lays waste to the group 31 within a couple of hrs. I switch over, start up, switch back and recharge. I cannot use 2 grp 31's because of space limitations. The only problem I ever had was last year. I was charging and I was also running the stereo up and it caused the 50A main in the alternator ckt. to trip. Oh...had some wires get a little too hot as well a while back!
I have to disagree with whoever posted "They dont like serious discharge cycles, and you will probaly end up changing them quite often." I was led to believe the opposite, that deep cycle batteries dislike shallow charges and thrive on a full to low charge, and then recharge cycle.
Anyway, in 1999, my boat tunes registered at 117db as opposed to the 114db @ the drivers seat of my challenger, in the beautiful, hot summer day. The canvas up makes it beyond something I can measure (or tolerate MYSELF (db meter max @126db)).

I cannot recommend that anyone reading this post attempt this with their stereo's at home or otherwise. Hearing damage can result and it may aggitate your significant other as well! Hmmm......what was that she just said?

Scott


In a hundred years, it isn't going to matter anyway.

 

[OperaHouse]

Don't forget the Peukert Effect. Given this is boating, you would think that has something to do with beer. Your labeled amp hours will not be what you get since these are usually at a very low draw to give the biggest number. As the current increases, the amp hours decrease dramatically. For example, I have a 105 AH battery that is also rated at 180 Reserve Minutes. RM is calculated at about 25A draw. So, you loose 30AH right off the bat. Adding a second battery will reduce the draw on each battery and increase the effective AH of each battery.

 

[Tunalover]

Thanks all! What a helpful forum!


Tunalover