Adding a battery bank

99 Posts
Dec 26, 2010 01:40 am
Adding a battery bank

I have a bank of six Surrette 4-CS-17PS 546Ah batteries in series for 24V, installed in 2008.  I would like to now double my capacity by adding a second bank of identical batteries in parallel to make 1092Ah.  I purchased a Blue Sea battery selector switch so that I can use both banks in parallel or disconnect one or both.  My question is what considerations, if any, are there in connecting the two banks together.  Do they need to be at a similar state of charge?  Will the stronger bank charge the weaker one?  Will it harm the batteries to suddenly parallel in another bank at a different state of charge?  Should I equalize both banks together?
351 Posts
Dec 26, 2010 03:43 pm
Re: Adding a battery bank

Do not use the selector switch.

Your decision is either that you are going to have one bank that consists of two strings (1092ah total) OR you are going to have two banks of 546AH that will NEVER be paralled.

If you decide to parallel them, the answers are:

Do they need to be at a similar state of charge ? Yes.

Will the stronger bank charge the weaker one ? If by "stronger" you mean higher state of charge, Yes.

Will it harm the batteries to suddenly parallel in another bank at a different state of charge ? Very possible. The greater the difference in state of charge, the more likely damage is to occur.

Should you equalize both banks together ? NO. One bank of two strings, yes. Two banks of one string each, NO.

Now the debate on whether to combine them or not, can start.
Should 2 year old batteries be combined with new ?

99 Posts
Dec 28, 2010 12:38 am
Re: Adding a battery bank

The intention is to always have them in parallel.  The switch serves several purposes.  First, it combines the two banks close to the batteries so that I have only one pair of leads going to my DC panel.  Second, if I needed to take one bank offline in an emergency, it allows me to do so while retaining power to my system from the other bank.  This provides a level of redundancy.  I'm not planning on switching banks on and off routinely.  It's just a convenient disconnect in case I need it.  Other than such an emergency, I can't see why one would want to disconnect one of the banks.
351 Posts
Dec 28, 2010 12:56 pm
Re: Adding a battery bank

In an emergency, taking the whole system down until you are 100% positive of what is going on, is the safest route. A hasty decision to try to keep power flowing is what caused the Three Mile Island accident.

But if you are going to do it anyway, you should be aware that the switch you intend to use, is not code compliant for residential use. There are knife switches and other disconnects that can be used for the same function, that have the appropriate UL listing.

2 Posts
Dec 29, 2010 11:02 am
Re: Adding a battery bank

As long as the two years old battery is in good operational condition, you should not notice much difference or negative impact. Industry best practice will however discourage you from doing this. But if you chose to proceed then;
1. Checkout each battery in the 2yrs old battery bank to confirm they are still in good condition.
2. If they are, then take them to full charge,
3. Bring in the new battery series string,
4. Connect the 2yrs old battery series string to a properly sized breaker in your DC panel.
5. Connect the newer battery series string to a another suitably sized breaker in the same panel.
6. Take your two leads from the positive and negative bus bars of the DC panel to your inverter input.
7. Switch on the two battery series string breakers one after the other and
8. Bring the battery bank now made up of two series strings connected in parallel to full charge. There is no need for the switch you are talking about as you can achieve your aim (individual string isolation for maintenance & over-current trip) with the DC breakers.
You should have no serious negative impact of the older sets of batteries on the new one. I have done this in the past and can tell you from personal experience that it works surprisingly well.
« Last Edit: Dec 29, 2010 11:12 am by Clinton Akpan »
99 Posts
Dec 29, 2010 10:52 pm
Re: Adding a battery bank

I have the Outback Flexware "breaker panel", which is to say very bare bones, and does not support multiple breakers of the size required for two battery banks.  I'm using the selector switch and two 200A class-T fuses instead.  Serves the same purpose.

What do you mean by "checkout each battery"?  Resistance across the cells?  Hydrometer?  I'm not sure what to look for.  My battery bank charges fine, I equalize it monthly, and I have a desulphator attached, so I assume they are in fine condition.  These are high-quality batteries with an expected life of 20+ years.
2 Posts
Dec 30, 2010 09:16 am
Re: Adding a battery bank

Yea! you're right. You can check the;
1. Internal Resistance - especially if you have the manufacturers specification or data sheet to compare with.
2. Specific gravity - (for flooded batteries only).
3. Open Circuit Voltage of each battery.
If the data sheet is not available, you can confirm these data by comparing them with same data (average of each) taken from the new batteries.
Generally, from what you're saying it sounds like you have nothing really to worry about as the old batteries appears to be in very good operational condition. Good luck!
18 Posts
Jan 5, 2011 04:28 pm
Re: Adding a battery bank

Also a 100A battery tester could be used to check the voltage of each battery under load.
99 Posts
Feb 14, 2011 12:55 pm
Re: Adding a battery bank

Here's my completed setup:

Just waiting on the batteries to be delivered now.

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