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Old 30-11-2019, 13:16   #1
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Connecting batteries

Hi, My house bank is 2 6v flooded cell deep cycle batteries connected in series for 12v 232 ah. I have a spare near new 12v 680 CCA 78 ah battery. Can I connect in parallel to the house bank or will the different battery types and capacity give me charging problems?

The second option is to connect in parallel to my start battery which is the same type of battery.

Any advice will be much appreciated. Cheers.
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Old 30-11-2019, 15:40   #2
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Re: Connecting batteries

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Originally Posted by Allan M View Post
Hi, My house bank is 2 6v flooded cell deep cycle batteries connected in series for 12v 232 ah. I have a spare near new 12v 680 CCA 78 ah battery. Can I connect in parallel to the house bank or will the different battery types and capacity give me charging problems?

The second option is to connect in parallel to my start battery which is the same type of battery.

Any advice will be much appreciated. Cheers.
Hell No! Lead acids connected in parallel should be same type, capacity and condition (age). Your 78aH battery will be destroyed quickly, which will then destroy your 232aH string.

By all means connect in parallel with your start battery if same type and capacity.
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Old 30-11-2019, 18:54   #3
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Re: Connecting batteries

Absolutely false.

It is when connecting in series that units should be the same capacity, health etc.

With parallel capacity matters not at all, and SoH no big deal as long as reasonably decent.

Of course one paralleled unit completely failing, as in e.g. internally shorting will bring down the whole setup, so some extra care with proactive capacity testing would be a good idea.

The same basic chemistry though, so voltage vs SoC are roughly parallel.

And of course the ideal is, all units in the bank match whether serial or parallel, all replaced at the same time.
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Old 30-11-2019, 19:13   #4
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Re: Connecting batteries

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Originally Posted by john61ct View Post
And of course the ideal is, all units in the bank match whether serial or parallel, all replaced at the same time.

I went to a 80 hour class dealing with Club Car golf carts back in the late 90s. They were very adamant about mismatching batteries as well as changing them out at different times. Both were considered a big no no as well as creating potential future problems.
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Old 30-11-2019, 19:49   #5
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Re: Connecting batteries

Well even professional experts hold and teach false beliefs.

But yes, if talking about savings of small money, doing anything less than optimal best practices is silly.

I do stand by my words, but admit they were written for the sake of propagating accurate knowledge, correcting members' errors, rather than as practical advice.

If we were talking about banks worth many thousands, and a technically proficient but poor owner, then maybe worthwhile.
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Old 30-11-2019, 19:53   #6
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Re: Connecting batteries

When connecting devices in parallel, the assumption is that all of the devices are "identical" and will share the load equally. Since not all devices are "identical," good engineering practice says that there will be something added to the circuit to help equalize the various paths. For example, when you use multiple transistors in parallel to increase the drive current, there will typically be a small resistor in series with each transistor. The same is true when driving LEDs in parallel.

You might be able to get away with connecting different batteries in parallel, but I think its asking for trouble.
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Old 30-11-2019, 19:54   #7
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Re: Connecting batteries

go buy 4 new golf carts if you want a bigger house bank. they are cheap.
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Old 01-12-2019, 00:14   #8
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Re: Connecting batteries

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When connecting devices in parallel, the assumption is that all of the devices are "identical" and will share the load equally.
What assumption?

Yes the usual case, yes best practice. But, they don't "need" to share anything equally, after a high C-rate period they all self-balance, same with a charging current rate, some units higher CAR than others.

> Since not all devices are "identical," good engineering practice says that there will be something added to the circuit to help equalize the various paths. For example, when you use multiple transistors in parallel to increase the drive current, there will typically be a small resistor in series with each transistor.

No, just adding points of failure for no reason.

As long as they're healthy and their chemistries have similar charging setpoint,

other caveats as stated above,

could parallel 20Ah, a 60Ah and 250Ah strings at the same voltage, no harm no foul.

They won't wear evenly, so extra care needed to retire units proactively as their EoL point reached.

Again, I'm not recommending this actually be done in practice, but it isn't "dangerous", just that that last is a bit complex and inconvenient, most owners don't have the gear nor skills anyway.


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go buy 4 new golf carts if you want a bigger house bank. they are cheap.
Exactly, for a small bank, even say 600Ah, we're talking nickels and dimes savings, just not worth the trouble.
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Old 01-12-2019, 03:09   #9
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Re: Connecting batteries

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Absolutely false.

I have recently purchased a 35 year old Bruce Roberts 45 with zero sailing experience - so I am here to learn and am grateful for the vast pool of knowledge allowing me to benefit from others experience.



However I have been an electrical contractor in South Africa for 30 years, the last 10 years since our esteemed president and his cronies destroyed our previously world class electric utility I have installed and maintained backup systems on a weekly basis, and so feel I may have something to contribute in your case.


The vast majority of maintenance calls I receive are as a result of battery failure, and the majority of early battery failures are as a result of poor wiring practice resulting in battery imbalance and the use of unmatched batteries.


So my best advice to you is DONT DO IT.
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Old 01-12-2019, 03:53   #10
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Re: Connecting batteries

All batteries in a bank must be of the same type (chemistry).
All batteries in a series bank must have the same capacity, or amp-hour rating.
All batteries in a parallel bank must have the same voltage rating.
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Old 01-12-2019, 05:49   #11
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Re: Connecting batteries

Connecting a different capacity batteries in parallel isn't the ideal in a perfect world BUT if all batteries are the same chemistry IE flooded lead acid and all are in good condition then:

- One battery or set of batteries will not grossly over or undercharge nor will any of them discharge dramatically more into a load than the others.

- While not ideal this arrangement will not cause the boat to explode nor the batteries to catch on fire nor will it cause the immediate, untimely demise of any of the batteries in the system.

- Over time as the batteries age this arrangement might result in a greater imbalance if one battery isn't charging well that could impact the maximum expected life of the rest of the batteries or cause the good batteries to drain into the bad one using capacity of the system
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Old 01-12-2019, 06:23   #12
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Re: Connecting batteries

So I did this for a few years with a starter battery left over from when my boat had a diesel and a deep cycle battery.

The deep cycle battery was the first to fail (after 4 years) and I'm thinking since the two were in parallel the starter battery would charge up faster and then the charge controller would stop the charge and go to float before the deep cycle battery was fully charged

I noticed later after I replaced both with identical deep cycle batteries that they took much longer to charge up. The two deep cycle batteries in parallel.
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Old 05-12-2019, 04:07   #13
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Re: Connecting batteries

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Well even professional experts hold and teach false beliefs.

I was agreeing with your advice.
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Old 05-12-2019, 07:24   #14
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Re: Connecting batteries

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Originally Posted by Bycrick View Post
When connecting devices in parallel, the assumption is that all of the devices are "identical" and will share the load equally. Since not all devices are "identical," good engineering practice says that there will be something added to the circuit to help equalize the various paths. For example, when you use multiple transistors in parallel to increase the drive current, there will typically be a small resistor in series with each transistor. The same is true when driving LEDs in parallel.

You might be able to get away with connecting different batteries in parallel, but I think its asking for trouble.
Male bovine excrement.
The batteries will share the load exactly in proportion to their capacity. It is a common misconception that if you connect two batteries in parallel that each one will provide 50% of the current. This is totally wrong you can connect a 10 amp hour capacity battery in parallel with a 100 amp hour battery and they will share the load 10:90. One battery can't discharge to a lower voltage than the other they are connected in parallel!

Think of it as two water tanks, 1000 gallon and 100 gallon connected to each other through a ground level pipe. They will both have the same water level (voltage) and as you add or remove water each will receive or contribute in proportion to its capacity.
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Old 05-12-2019, 07:41   #15
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Re: Connecting batteries

If the 78AH battery is nearly new replace the engine start battery with it. If the engine start battery is also nearly new, then just sell it. A 78AH car battery isn't going to do much for your house bank capacity and faffing around keeping it on the boat but not connected with occasional charges isn't great either.

Less is more, sell it and put the money in the piggy bank ready for your next change of batteries.

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