Ganging 12v SLAs: diodes to prevent "cross drain"

[waoj]

It seems to me that if you have a bank of batteries in parallel, and just one of them fails in 'the right way', it could ruin the value of the whole chain.

Specifically, if one battery starts converting energy to heat, it will drain energy from the others faster than the trickle charger can keep up.

I have a UPS in the basement who's batteries weren't lasting as long as I would like.  And I had some spares at work, so I brought some home.

None of them are particularly new.  But my thought at the time was that if they were built to have 8AH, and were only performing 4AH of capacity today, that by adding another pair of similarly shitty 4AH batteries, I could achieve 8AH of total capacity. 

But instead, I noticed my runtime decrease even more (from 8 minutes to 4)  And even after I undid my changes, it stayed around 4.  Usage is low.  I'm guessing around 100 watts to run a decent network switch and some wifi gear.  It states 865 watts as "nominal", and current load 11 percent

Maybe it's a coincidence.  Or maybe I ruined my original cells by introducing other crappier cells.  I let them acclimate for a few days before the first test, and recharge together before removing them and doing a second test without them.

Is there a way I could use diodes to prevent this problem?  Or (as I suspect) would any arrangement of diodes that prevented cross drain also prevent .... charging...

My UPS is 24VDC internally, using two 12v SLAs in series.  So I made a harness that extended two 10AWG wires out of the UPS to add externals.  Though, now I'm thinking I want to just get a couple car batteries and replace these little SLAs for good.  I wish the thing was 12v internally to be simpler.

I also have an APC symmetra in storage but I worry it's own power consumption will cost more than a couple batteries each year. and it uses cartridges of 10x 12v SLAs, so that's a lot of money for essentially, a gamble, and its way overkill for the load I have.  Even if I add a couple hundred watts putting the TV on it, its still overkill.  I guess I need to shut up and go meter it out to know for sure though on the symmetra.  But any chance I can use diodes to get the last bit of usefulness out of old SLAs.

[james_s]

If one of the batteries fails you should replace it. Adding diodes for isolation could work, but it will introduce additional losses. Even Schottky diodes will have a drop of a volt or more at high current.

Car batteries are designed to supply very high current for a very short period for starting a car, they will not hold up well to UPS duty. What you want are deep cycle batteries, they're used often in boats and RVs and will tolerate that treatment much better. The tradeoff is the amp hour capacity is lower.

[wasedadoc]

If you use diodes to isolate the batteries from each other they cannot be recharged by the UPS.