looking for multiple battery protection design.

[JimPghPA]

I have a friend that uses a quantity of 10 lead acid 12 Volt batteries in series to power a big back up power supply system. The problem he has had is that when one of the batteries fails the charging system over charges the remaining good batteries and destroys them. He is looking for something that prevents this from happening again if a battery fails with the new batch of batteries.

I am thinking of using an LM723 wired to trip if the voltage across the battery it is monitoring goes above a preset voltage like 14.2 Volts, and that would drive a light emitting diode of an optio coupled output transistor. There would be one of these circuits across each battery and the output transistors would be tied in parallel so when any one tripped it would shut down the charger.

I am looking for any suggestions on doing in a better way, and also if you think 14.2 Volts is the proper Voltage to set this at.

What would be real nice is an existing system that already does this. I do not need to reinvent the wheel if there is already wheels out there to purchase that would require less of my time.

Thanks for the replies in advance.

 

[supton]

I might think to find a high bit count ADC and then feed it the 9 voltages present (14V, 28V, ...140V). Run that into an Ardunio. The Ardunio could then scroll through what voltage is on each battery, light an LED in case of fault, and have one bit that drives an appropriate opto/relay/whatever that would trip out the charger. The ADC design might be a bit tricky, what with 140V, but it's not high speed here so high ohm resistors would be fine. Sampling rate doesn't have to be very fast, like read one battery, wait a second, move to the next.

Bit intensive, but, code can be easier to adjust than hardware. That and making 10 of any widget tends to be more work than making one "big" widget.

That out of the way...

LM723, that's a regulator. I'd pick a newer comparator; might be able to find one with an internal reference that you could divide battery voltage down to and to compare against. Use the comparator to light off an LED. Also have it drive a floating opto. Then, on the output side of the optos, put all ten in series. Design so that in the good case, all opto's are on. If one battery faults out, it turns off its own opto, but then that kills the series connection. Idea being, the charger is somehow controlled by this string of opto's. [I'm saying opto's, it could be photomos or relays. Or put all opto's in series and use that to drive a transistor that in turn connects the charger to the batteries--just don't forget flyback diodes.] If a battery goes shorted, no power to run the op-amp and thus the opto is inherently off. I guess you need a dual comparator, for over and under voltage...

One issue with many of the cheapo older comparators is that they are open collector and while that can be made to work I think a full drive would be easier to work with here.

 

[mk378]

Charging a big series string of batteries from one high voltage source usually requires an active circuit on each battery to shunt some current around it when it is about to overcharge, so that the other ones can continue to charge. Especially if this is some motly assortment of batteries not a matched set.

The TL431 is good for this sort of voltage detect and shunt. It would of course need to be backed with a power transistor.

 

[JimPghPA]

The A to D approach is an interesting idea. I am not familiar with Ardunio but will look that up, and seriously consider that approach.

The LM723 has a stable Voltage reference output and a comparator that could be used to compare the reference Voltage to the battery voltage, and an output transistor capable of driving an led of an optio isolator.

 

[JimPghPA]

I looked up the Ardinio. A similar system with 10 or more analog inputs would be better for this application.

Does anyone know of anything similar to an Ardinio that has 10 or more analog inputs?

 

[andyd]

Would adding another battery to the series help ? What kinda back up doesn't have all sorts of feed back alarms. Any trouble with UPS dispatches a tech automatically in the voice+ data world

 

[djb]

This is a common use case. Look up BMS (Battery Management System).

A full BMS will also include the ability to balance the individual units. That's done by slightly draining the highest charged unit, or providing addition charging to the weakest unit. Some systems for larger packs even transfer charge between cells using switched capacitors as a way to balance faster without excessive heat. The smartest units track the state of charge and cell characteristics to do a combination of "top balancing" (balancing when almost all units are fully charged) and "bottom balancing" (remembering the weakest unit at full discharge and making certain that it gets extra charge).

The challenge with designing a BMS is that each monitored unit in a battery stack is at a different voltage. Monitoring a large pack quickly gets to dangerously high voltages and measurement inaccuracy. The usual approach is using a few modules, each handling a connected subset of units, and having them communicate using some isolated scheme. As you might expect, these quickly become expensive.

The easiest DIY approach is a voltage monitoring board with a Zener diode, resistor, visible LED and optocoupler on each 12V battery. Wire the Zener, LED, optocoupler LED and resistor in series. Wire the isolated optocoupler output to a central monitoring board where it light a second LED or be "wired OR" to a warning LED.

 

[ironman_gq]

http://www.chargingchargers.com/quick-charge/mb1210x10.html

He needs a 10 bank charger to charge each individual battery on it's own. I'm sure there is one out there that has battery monitoring and will alert you to a bad battery. A 10 bank charger at least will eliminate the issue with a dead battery destroying the whole bank and is generally the correct way to charge a series connected battery bank.