Battery Management Systems

[Demian]

With all these open source projects... It seems the only two things we are missing are batteries and battery management... I'm not capable of initiating a conversation about planning open source DIY batteries, but I do believe that a BMS is important.

I am no expert on this subject, and much of what I think I know may be wrong. Please don't hesitate to interject and/or correct.

The three types of BMS I know of are:

1) Passive and isolated, like a zener lamp. At a certain Voltage current is shunted to a resistor (a bulb in the example i read). Limits overcharging.

2) Passive but interactive. I can't remember the example of this very well, but it sat in between batteries and shunted current between the two batteries until they are equal in voltage. IIRC it was a passive device, not actively moving current.

3) Active. like Lee Hart's (BalancerLand - Intro) I see a couple ways of doing this.

a) have a microprocessor watch voltage (and temp) of each battery and move current from the highest to the lowest etc.
b) have a microprocessor watch voltage (and temp) of each battery as it charges and remove batteries from the string as they reach a defined voltage for that charging phase
i) this would require a very smart charger or that the BMS work hand in hand with the charger. (you do have to reduce voltage when you remove a battery right? you can't charge a 24volt pack with 120 volts?)
ii) technically this isn't BMS, but if you are charging a pack every night and every night it is equalized, it's darn close to BMS (until a battery goes bad).

What I want out of a BMS:

Effective
inexpensive
average joe can build it

I would love for it to actively display individual voltage readings to a backlit dashmounted lcd, but I wouldn't pay an extra $100 for it.

[Ryland]

There was a guy at the Midwest Renewable Energy fair that had a home built charger that charged the bank of batteries then turned on a 12V charger that toped off each battery in the system, I've looked in to getting a pile of 6 or 12V chargers so I can charge each 6V battery so that it gets charged without having the affects of the other batteries in the string.

[bennelson]

Here are a few photos of the system Ryland just mentioned.







A logic board controls all those relays that route power to each battery individually to top it off. I seemed like a clever system.

Also, my friend Tom, who is building the AC Dodge Neon, is planning on having a custom battery monitoring system. That is going to be a while before that's done, but it may actually become a product to be sold.

If he goes open source with it, I will share the info here.

[Demian]

If you are going to design something to turn on a trickle charger, why not include the trickle charger in the design and not buy a dozen of them?

Again, I am no expert, but when you bulk charge you keep the constant voltage and when you trickle charge (or top off if I am using the wrong term), don't you keep constant (and low) current?

couldn't we do similar with PWM (since that is the fad)?

[rmay635703]

Originally Posted by Demian

If you are going to design something to turn on a trickle charger, why not include the trickle charger in the design and not buy a dozen of them?

I believe the purpose of having ONE charger switch and top off EACH battery was so they only used one charger to do the bulk then topoff charges by using electronics to switch and select the battery to be topped off, I would assume float would come after or during each battery topoff?

[ev59rag]

Here is one...

This is the one Im planning on using (dump controller) on my ev. Already using it for my solar panels. Works great.
ghurd - Hurd Solar Renewable Energy | Dump Controllers


Another similar setup but designed in tune for EV users like us.
Volt Blocher


And here for monitoring your batteries.
http://www.evconvert.com/article/led...monitor-part-2


Hope this helps.