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

usually the smaller battery is used to start you engine and the 130 amp would be a dedicated house battery

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

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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?

So I'm guessing the 40ah battery is a starter batter, and the 130ah is a house bank battery, in which case the best way to connect them is to connect the alternator to the house bank, and then charge the starter battery with a voltage follower such as the Xantrex Echo Charge or the Balmar Duo Charge.

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Sectorsteve said..
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?

No, as you have described connecting them in series, so that will give you 24 volts.

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Sectorsteve said..
I do also have a battery switch onboard that came with the boat i could install

That will be useful to turn off the equipment that is not permanently connected to the house battery.
It is necessary to have fuses or circuit breakers inline on the positive of every circuit, with the possible exception of the starter motor. As with the starter motor since you are holding a button (or turning a key) to activate the starter motor, if it shorted, you could release the button. A short circuit that is not protected by a fuse and/or circuit breaker will create a fire.

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

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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 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.

Both batteries are brand new but they are different makes and different capacities. They are both deep cycle batteries

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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

I wouldn't be game to connect 2 different batteries in parallel. If you really want to use both, then keep them electrically separated and maybe use different solar panels for charging each battery. Use the 40AH battery for lights and maybe the 130AH for heavier loads like the fridge.

You can buy electrical isolation systems for charging batteries you don't want connected in parallel after the charging is completed, but they are usually just high current diodes that result in a 0.6 volt loss in the charging level.

Batteries that are connected in parallel need to be identical. If one has a lower "Fully Charged Voltage" then the other battery will discharge into it when the charging system stops (sun goes down). My thoughts anyway.

Identical means same brand/model and AGE.

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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.

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HG02 said..
usually the smaller battery is used to start you engine and the 130 amp would be a dedicated house battery

www.supercheapauto.com.au/
Id add one of these so the system could charge both batteries
I fitted one of these link below to the above cicuit if bacically manages two battery banks and chargers either roughly speaking
www.bluesea.com/products/7611/BatteryLink_Automatic_Charging_Relay_-_12V_24V_DC_120A
Basically what Ramona is saying

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

Um so im still not sure. I do have a fridge. Drains a bit of power.
So do you think the 40ah could just run the tiller pilot only?
At the moment ive got 2 fat cables running from the OB Through the rear lockers and they connect directly to my battery terminals. The 40watt solar is wired through the control panel.
I have 2 plugs on the boat for solar. 1 in the cockpit and 1 in the forward cabin. Currently my panel is on the cabin top and plugged into the cockpit plug.
When the 160w comes(built in mppt like the 40w) this is being mounted on the Bimini frame over the transom and will probably connect directly to the control panel.
Im still not sure how to use the switch. Looks like i should keep the 40w separate. Can i connect the 40w directly to the control panel brass bars?

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.

Thanks again. The 40ah is 2 months old. The 130 ah is 2 weeks. So theyre both nearly the same age and in good condition. The 40ah seems to have liquid in it and you can top it up with water the 130 ah is completely sealed. Theyre both deep cycles. I guess i could try the parallel this week and keep and eye on them daily.

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.

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

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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

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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.

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.

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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 absence of significant external charge, the battery with higher voltage will charge the other one.

the reason for using batteries of the same chemistry and similar age is so that their charge characteristics are similar. also, with the batteries in parallel you want to check them regularly, as one battery going bad can then ruin a remaining good battery.

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.

Felix, I have two batteries of similar size connected through a rotary switch so I
can use one or the other or both. One is the house battery and the other the starter.
Often when I'm motoring I will charge both together. Is it true that if the cables from
switch to batteries are different lengths, the battery on the shorter length will charge
first ??.

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!

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QLDCruiser said..
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!

Thanks QLDC, I didn't know that.

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

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felixdcat said..
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.

What you say is true if connecting in series. It's not true when connecting in parallel.

Connecting in parallel is safe, and often done. There is very little danger in connecting batteries of different capacities in parallel.

The only issue is if one of them starts to go bad, it can pull down the good one. Regular maintenance/inspection can prevent this.

Another option is to use an automatic charging relay like this one: www.powerstream.com/battery-isolator.htm
This means that if your house battery goes flat, you can still start the engine. Again, this type of circuit is very well understood, and often implemented.

Here's a good diagram of best practices:
www.bluesea.com/articles/58

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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.

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Karsten said..

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.

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.

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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.