Connecting 2 batteries

Recently bought a big battery 130ah and i disconnected my 40ah and stored it but im thinking i should connect it also. What is the best way?
I just saw a diagram where theres a lead between the 2 batteries from neg to postive and then leads from the 2 leftover neg/pos on each battery to the back of control panel. Would that be ok?
I do also have a battery switch onboard that came with the boat i could install

I dont need a battery to start an engine. I just thought they could both be in use and get charge.

Hi Steve

if you do it that way you will be putting 24 volts to the control panel

if you connect the neg and pos together on each battery you would essentially have one large 12 volt battery and the voltage of both would equalise but I am not sure how the charging system would deal with charging dissimilar batteries

if you do it through a 1, 2 or both battery switch you could select either 1 or 2 with the engine running and the charging system would charge each battery independently

Regards Don

Thanks for the replies. No starter motor and no alternator.
Presently i have a panel. Lights. Depth sounder, stereo, bilge pump , dc socket, inverter, nav lights all with fuses on the panel. For charging ive only got a 40watt panel but about to get another 160watts of solar - so ill have 200watts of solar. Also my outboard is connected to the panel and charges batteries very well also. I recently bought the 130ah battery and swapped it out with the 40ah battery which is disconnected. I may add a tiller pilot and vhf soon to the list of things that drain power but ultimately im trying to keep things simple and not drain much power.

Select to expand quote

Karsten said...

Sectorsteve said..
I dont need a battery to start an engine. I just thought they could both be in use and get charge.

In that case, provided they are both of the same chemistry (flooded vs gel vs AGM) and both are in very good condition, you could connect them in parallel - ie. link the 2 batteries' positive and negative terminals with thick wire that you should keep as short as possible. It will just give you a bigger house bank at 12 volt.

Thanks Karsten . This is what id like to achieve. I just dont want my near new 40ah sitting around not being used. If it could add to my power bank and get charged with its big brother that's great

Select to expand quote

Sectorsteve said..
Recently bought a big battery 130ah and i disconnected my 40ah and stored it but im thinking i should connect it also. What is the best way?
I just saw a diagram where theres a lead between the 2 batteries from neg to postive and then leads from the 2 leftover neg/pos on each battery to the back of control panel. Would that be ok?
I do also have a battery switch onboard that came with the boat i could install

You already have a battery switch, presumably one that's a rotary to handle two batteries, just use that. Wire up the batteries as you normally would for a two battery system and use a different battery each day. Spend the money that the Xantrex Echo Charge or the Balmar Duo Charge would cost on two solar panels and controllers and have each one charge one battery. Wire the rotary switch so that the engine alternator charges the battery in use when the engine is running. You can use the switch to run both batteries at once if needed but best to avoid if possible. In about 5 years time change the 40ah for another 130ah and you will have exactly the same system I use.
Unless you suddenly want to install a fridge in your Tophat this will be more than adequate power.

I went crazy and paid $13 each for two solar controllers. Sectorsteve already has the switch.

Provided that the two batteries have the same chemistry there is no problem in connecting them in parallel. they will charge and discharge at the same rate regardless of capacity. But if they are different ages then the internal resistance will not be the same and the older battery will be slower to charge, will draw current from the newer battery and will not contribute to the overall capacity as it should.

If you are going to go ahead and parallel them, then make sure they are both fully charged before making the connection. If one has a higher terminal voltage then the other, then the one will the higher terminal voltage will discharge into the other. Be careful.

Select to expand quote

Sectorsteve said..
Crusoe i wont parallel them. Its too risky i think and they cost alot of money. When i figure out how to use the switch ill so that.
Would each battery go to the switch then neg and pos from the switch to contol panel

The negativity side from all batteries can go direct to the engine chassis in your case with an out board
Only the positives cables from the batteries go to the Isolator switch

When charging or discharging bat. bank, it is irrelevant what size, age or type are batteries.
The only time, you have to take care of system, there is no charge or discharge and all batteries
are connected together . They start discharging each other, due different chemistry.
I have 2 solar panels connected together and each bat. is connected through Schottky diode.
I use this for decades, after bad expirience with failing, misbehaving electronic.

I am a sparky, I would not connect 2 different batteries in parallel or serie!
The weakest battery will have more internal resistance and diminish the performance of the strongest one.
It is a bit like the law of the weakest link that limit the performance of the chain.
The battery with the biger internal resistance could get very hot.

Be careful using 1/2/both/off switches. If you accidentally turn it to Off while the engine is running, you can damage the alternator or wiring. Easy to do if the switch is located out of the way and you're doing things by feel.

Don't ask me how I know this!

It's usually printed on the switch in quite large letters

Select to expand quote

fishmonkey said..

Karsten said..

PhoenixStar said..
... the older battery will be slower to charge, will draw current from the newer battery...

Not sure I follow.

Notwithstanding what the OP chooses to do, I'd like to understand this issue.

If the two batteries' positive and negative terminals are tied together with short, thick wires, their voltages will always be exactly the same.
Current will only ever flow from a high to a low voltage. If the voltages are tied to be the same, no current will flow.

For the sake of our collective understanding, if I'm missing something here, I'm happy to learn.

...connecting them doesn't magically make their voltages the same. the voltages of the batteries will equalize and settle at an intermediate point (the physics of it isn't a simple equation)....

In the nicest possible way, I must disagree.

I'd like to clarify this because there may be Seabreeze forum members/visitors that read this thread in future looking for info on the parallel connection of house bank batteries - it's a common question.

Excluding the moments after initially connecting the positive and negative terminals of two flooded SLA's together (parallel) with a short, thick cable, the voltages will thereafter be exactly the same, no matter internal resistance differences or how many charging-usage cycles they go through.

The combined voltage will not be an "intermediate point" voltage, but for all practical purposes the same voltage - eg. if one is 12.6v, the other will be 12.6v. If one is drawn down to 12.2v, the other will also read 12.2v - it has no choice.

Reason: Resistance of reasonable thick copper cable - for example, gauge AWG 4 at 400 mm length (and AWG4 is not all that thick)- is approx 300 micro-Ohm. Application of Ohm's law shows that the two batteries will never maintain a voltage differential across such a minute resistance.

Select to expand quote

PhoenixStar said..


Yes, and so they will charge and discharge according to their capacities, so the combined amp hours of batteries in parallel will be additive. only problem with parallel batteries is when one battery fails, particularly with an internal short. And that can be a problem even with single batteries.

Indeed, if one 12v battery happens to short out one of its 6 cells, it can lose about 2v and thus end up at around 10.5v. Yes, then both paralleled batteries will measure 10.5v.

Admittedly internal shorts seem to be less prevalent than sulfation with typical house bank use. Over the years I have personally lost about 3 house batteries to sulfation (gradual capacity loss) but never had an internal short. Everyone will of course have his own experience of how often he gets internal shorts.