DC cross bar for battery testing

[Billy Croan]

I've got a new pet project.  I'm managing a very large UPS that will be coming due for battery replacement soon.

12v Batteries are in 'strings' of 8 for a system DC voltage level of 96 volts.  If any battery in a string is faulty, the whole string is rejected by UPS and disconnected from the DC bus.

I'm thinking instead of replacing every battery in a string, I'd like to only replace the worst ones and get another year out of the remaining cells.

Of course, you can't just put one new battery in the bad battery's place.  But the reason behind that isn't ageism....  It's that they need to have about the same internal resistance and capacity in a string to charge and discharge evenly.  If they charge a discharge unevenly they melts.

So I plan to serialize inventory and never mix two generations of batteries in the same string.

Further, I'd like to test each battery if there's no physical damage or deformation.  A discharge and charge, perhaps multiple cycles.

And I need to automate this for the most part, to avoid repetitive work and eliminate error.  

I think I want to have a thermal sensor on top of which I set the batteries.  Ground them to the shelf they're on, and have each battery connect through a current meter to a relay that can connect then to a drain or a charger.

I want to monitor voltage at each cell and current in and out.  Then retain a charge profile of each battery that consists of the temperature, voltage and current during each charge and discharge cycle recorded once per second.  From that I'll calculate a score of the battery's "health".

When building a string of batteries, we will make sure that all batteries in the string are within 5% or less of each other's score.

I want to do multiple tests unattended, and kind of want to avoid having to buy a separate charger and discharge element for each battery.

I'm thinking this might be more scalable if I get one powerful supply, maybe 14 volts, (maybe I should get something adjustable) and put a big space heater outside the building to act as a drain and then connect the batteries to one or the other.  Perhaps monitoring current at the charger and heater as well so as not the overwhelm them.

I think then, effectively, the system that I would use to connect and disconnect each battery from one of the two available buses (charging and discharging) would be called a cross bar.  Right?  

How should I construct a cross bar like that?  Other than...... Carefully....  What components!

Each battery is rated at most 9 amps, and I could imagine doing either 4 or 8 in one go.

I was also thinking perhaps I could use energy from the cells in discharge to charge the other's.  Basically using a boost converter/jewel thief until a cell is empty and then using a charger to compete the charge.

That way I'm not wasting as much energy and don't need a resistive load bank.  Then again a 12v 10a jewel theif might cost a lot more than the wasted energy....

What I'm I overlooking?

How would you build a crossbar like this?

I'm thinking an spst relay to detach/attach each cell, and a spdt to choose which bus, so no software error can ever cross the buses.

It's not hard to find cheap 10a relays, but what factor would you overbuild something like this? Maybe not much and just put a fuse at each battery, switch to a breaker if I end up replacing them often.

It's in a commercial/concrete/steel area so I'm not terrified of a cell dripping.  I really don't think fire would go anywhere.  We prohibit combustible materials.  And I'd terminate the test if current or temp got too high.

Anyone ever do this before?