Upsizing battery bank

[psish]

Hello,

I am facing an issue and I can't seem to find a solution to. I was given a very simple solar kit including:
- 1x Superwatt MPPT Inverter/Charge Controller 3KVA 25A
- 2x 12V 100Ah Superwatt GEL Batteries
- 2x 330W 24V panels

It's been used only a couple of month in an off-grid location, and I'd like to upgrade the kit to double the storage and panels (go up to 1.4kW panels and 4.8kW storage).

Unfortunately, the solution of just "getting rid" of the old batteries - since it's not a good practice to pair batteries of different age/usage - bothers me a lot as they are really recent (bought a year ago, plugged in 6 months ago).

I would love your input on any solution to upsize my battery bank without throwing away the 2x 12V 100Ah batteries in my possession.

Here's the things I thought of:

1. Wiring the batteries as such, trying to pair them by "age" in parallel, and then the two groups in series, but that doesn't really solve my issue. The newest batteries will degrade to the level of the old ones, but I am not sure how bad it is, since the old batteries were used only 4 months or so... How badly is it going to perform, and how long will the bank last doing so.



2. Using a 2-source battery master switch, so I can switch to one or the other battery bank when one group is fully charged or too low to draw power from. But the manual switch really isn't optimal for me.

3. Finding a way of an "automatic" switch that does that for me, charge the other bank when one is full, draw power from the two banks depending on their charge... But I can not find anything like this that exists. Any idea ?

4. Use the old batteries on an independent 12V system for USB/Lights, but that means having a dedicated panel and charge controller wired to it, and I would love to avoid it.

5. Re-sell the current bank and buy a bigger one...

6. Any other idea that you guys could think of?

What do you guys think? Thank you very much in advance.

Cheers,
John

 

[DerpsyDoodler]

Your wording is a bit confusing but based on your diagram it looks like you've been using a 24v system, so here's the advice I'll offer:

First, your current battery pack is not that old.

Second, do not mix different aged batteries in series. Keep the original set as is, add a new set in parallel.

1. Fuse each series string separately.

2. Install with a disconnect that will allow either one, or both to be connected simultaneously.

3. Balance the wiring so they share load equally.

4. Monitor/maintain the health of your batteries.

What does this achieve?

It keeps similar-aged batteries isolated to each other. Protects either from sudden discharge of the other. Either string can be taken offline for maintenance while allowing the system to continue functioning.

 

[psish]

Your wording is a bit confusing but based on your diagram it looks like you've been using a 24v system, so here's the advice I'll offer:

Forgive me for my wording, English is not my mother tongue, and sorry for the confusion, yes I have a 24V system!

First, your current battery pack is not that old.

Second, do not mix different aged batteries in series. Keep the original set as is, add a new set in parallel.

Thank you very much for your advices. I am having troubles visualizing the wiring you propose. Do you have a diagram I could find somewhere online for this wiring?

1. Fuse each series string separately.

Roger!

2. Install with a disconnect that will allow either one, or both to be connected simultaneously.

Yes that was one of my ideas, but I hope that I don't have to switch it manually to one bank or the other, hopefully I can use both banks most of the time?

3. Balance the wiring so they share load equally.

Sounds good.

4. Monitor/maintain the health of your batteries.

Should I monitor both sets of batteries with its own BMV-700 ?

Thank you very much!

Cheers,

 

[psish]

Would the wiring look something like this?

 

[acdoctor]

Rolles Surrett advises replacing a problem battery in a group without problem / concerns in the first 6 months. If it were me I would add to them very soon. Best wiring same age or not is the second method you described.

 

[psish]

Rolles Surrett advises replacing a problem battery in a group without problem / concerns in the first 6 months. If it were me I would add to them very soon. Best wiring same age or not is the second method you described.

Great, thank you!

So do you guys think the single BMV-700 is enough/good to monitor the to groups?

 

[DerpsyDoodler]

Great, thank you!

So do you guys think the single BMV-700 is enough/good to monitor the to groups?

One BMV, yes. When I said monitor/maintain, I meant something like:

At the very least, once a month you should check that the series string batteries are staying balanced. After a full charge put a multi meter on each one to see what their voltages are. you may need to balance them occasionally.

Make sure you use equal length cable from each string (as well as for the series connections) to keep the load balanced.

Finally, you can find a disconnect switch that will give you the "single/both/single" circuit functionality. Here's an example:

Attwood 14230-3 Single Pole 4-Way Selector Marine Battery Switch https://www.amazon.com/dp/B0000AZ7X6/ref=cm_sw_r_cp_api_i_HEH2B80T7SXARN59YV4F

 

[psish]

One BMV, yes. When I said monitor/maintain, I meant something like:

At the very least, once a month you should check that the series string batteries are staying balanced. After a full charge put a multi meter on each one to see what their voltages are. you may need to balance them occasionally.

Make sure you use equal length cable from each string (as well as for the series connections) to keep the load balanced.

Finally, you can find a disconnect switch that will give you the "single/both/single" circuit functionality. Here's an example:

Attwood 14230-3 Single Pole 4-Way Selector Marine Battery Switch https://www.amazon.com/dp/B0000AZ7X6/ref=cm_sw_r_cp_api_i_HEH2B80T7SXARN59YV4F

Amazing, thank you so much, you guys rock!