We've moved house and are rebuilding our off-grid system at the new place.

We own two banks of 12 * 2V  lead acid gel cells and would like to switch seamlessly between them. They have different capacities. We want one set charged, then charging switched to the other, and at night one set used, according to SOC, then switch to the other.

Has anyone already done this and can share their insights?

Can this be done with off the shelf equipment or do we need to build computer control eg with an Arduino?

We were using a Plasmatronics PL40 for SOC but don't intend to reuse this in the new setup. Which battery monitor is suggested. I'm looking at those from Victron.

"and would like to switch seamlessly between them"

What do you mean by this? Will you have a changeover switch that you switch or do you want to use a contactor?

Not a manual changeover switch but an automated system. 

Start on one battery bank, for a brief period it needs to have both banks in circuit, and then switch out the first bank.

Or do you have any better ideas?

How often are your batteries pressed into service? Daily (or nightly?)

 

7 hours ago, drlizau said:

Not a manual changeover switch but an automated system. 

Start on one battery bank, for a brief period it needs to have both banks in circuit, and then switch out the first bank.

I was thinking to do the same, to switch between two battery banks. When one bank’s voltage gets low switch with a contractor to the other battery bank. I was thinking to connect both battery bank’s negatives together as a common, then use a change over contactor to switch only the positives between the two banks. The only problem might be with the positive dc switching, there might be a couple of milli second switching time that might switch the inverter off and on again but that I don’t want to happen.

This is the response by my bud in the US who has a lot of experience in this business: (excuse the size of text!)

There is allays someone trying to do something like this to no end. Putting them together means less discharge per cycle making them last longer. separating them only shortens their life. bad idea.

The battery banks have different capacities, so joining them together is not a good idea either.

They are used every night.

Edited February 2, 20206 yr by drlizau
addition

Check out these comments on 4 x 4 Community forum: https://www.4x4community.co.za/forum/showthread.php/323320-Connecting-backup-batteries-of-differing-capacity-s

Add another 24v inverter. And run both inverters. If you need, split what needs power between the two.

On 2020/02/02 at 9:56 AM, drlizau said:

Not a manual changeover switch but an automated system. 

Start on one battery bank, for a brief period it needs to have both banks in circuit, and then switch out the first bank.

Or do you have any better ideas?

I have been thinking exactly that an automated system that switches between the two banks

 

Once one bank is depleted the system automatically changes to the other already charged bank , at the same time puts the depleted bank on charge

 

Then from there repeats the operation leaving you using a charged bank at all times 

And

A bank always on charge

 

There must be a product for this

 

Thanks

Mr rsc

Why not just parallel the banks?

Edit: as for auto switching, a sonoff feeding a contactor should probably work.

Edited March 16, 20215 yr by Speedster

On 2021/03/16 at 8:17 PM, Speedster said:

 

Why not just parallel the banks?

Edit: as for auto switching, a sonoff feeding a contactor should probably work.

 

One thing to keep in mind is the dangerous arc that is created when switching DC under load, a proper DC contactors should probably do the trick if rated for the correct load. I would think a common could be created by connecting the negatives of different banks together and have changeover only between positives of different battery banks. It would be interesting to know if someone has attempted this successfully between lithium iron and lead acid banks.

9 hours ago, Gerrie said:

One thing to keep in mind is the dangerous arc that is created when switching DC under load, a proper DC contactors should probably do the trick if rated for the correct load. I would think a common could be created by connecting the negatives of different banks together and have changeover only between positives of different battery banks. It would be interesting to know if someone has attempted this successfully between lithium iron and lead acid banks.

For a 24V system you shouldn't have an arcing problem.. 

I haven't seen many (single pole) DC contactors for large currents but I'm sure they exist..

Edited March 19, 20215 yr by Richard Mackay

On 2021/03/17 at 5:17 AM, Speedster said:

Why not just parallel the banks?

Edit: as for auto switching, a sonoff feeding a contactor should probably work.

Our battery banks are not paralleled because they are of different sizes and different ages.