BMS for SLA-s

[vax]

Is there some schematics for charging SLA-s more intelligently? The thing is, I have battery pack consisting of 4 identical 12V SLA-s. When they age, they will be charging uneven. One battery is full while other still needs some charging. OK I could connect them parallel and charge with 12V, but that's a bit low-tech way. Or buy four 12v chargers? Again somehow silly...

 

[pgt400]

Single chargers or a 4 output Bank charger are the ways to go. I have seen that some have tried using (2) 6.8 volt zeners and a 5 ohm resistor in sieres across each cell but never heard any results on how well it worked

 

[Russell]

There are a number of articles on constructing zener diode shunts for lead-acid batteries simply search on "zener balancer" or "zener regulator".

Here is one guy who used them and one video;

http://a4x4kiwi.blogspot.com/2008/06/battery-balancing-cheap-way.html

http://www.youtube.com/watch?v=tioCY0v_UHM


I ordered 6.8V zeners and some 5 ohm 2W resistors from Digi-Key to use on my 36V SLA pack consisting of (3) 12V-9AH batteries. I have not installed them yet. One reason is my batteries are very well balanced already. I checked them and they are within a few hundredths of a volt from one to the next. Another reason is the zeners I received actually test out at 6.6V and while this is within tolerance it would produce a 13.2V shunt which is a bit less than I'd prefer. And lastly my charger goes into float mode (41.5V or 13.8V per battery) when the batteries are charged as a smart charger should.

I suppose if your batteries are older or mismatched or you are using a "dumb" charger the shunts would probably help. For me they seem unnecessary though I will monitor the individual batteries over time to see if they remain as balanced as they are now.


-R

 

[D-Man]

I've found when they are get older, the time from green light to 0.0 amps fully charged goes down. When I mixed a new smaller 9ah battery in with my (2) 3000 mile 12ah batterys with reduced capacity. The 9ah takes much longer to charge fully during the float process when it was new. I don't feel its a big deal to mix it in like this with 2 old batterys. Got almost 1000 miles on the 9ah battery. With an amp gauge hooked up to a powersupply or 12 volt charger you can check the batteries individually to see if their charged and then use a stopwatch to see how long it takes to charge up all the batteries with the main charger. If you want balancers for sla's, they do make "powerchec equalizers" to keep a string of sla's balanced but if something goes wrong, I think it might just juice the whole pack.

 

[fechter]

The simplest route is to just use individual 12v chargers.

For large strings, Rudman regulators seem to be the standard for EV's
http://www.manzanitamicro.com/parts.htm

You could build your own power zeners as mentioned above. I'd be inclined to use a LM431-based voltage reference so you can make the voltage adjustable.

Powercheq battery balancers are another option, but somewhat expensive. You would need 3 of them for 4 batteries.

 

[Eric G]

I've got a 36 volt / 7.2 ah sla pack and I charge the 3 batts in parallel with a 3 stage 12 volt smart charger after a year and ten months they're still within 2/100's of a volt of each other.I'm really getting limited range now (3 or 4 Km's) so I'll have to get new batts.If I get sla again I'll charge the same,seems to work fine with no balance issues.

Eric

oops forgot to say I use a 36 volt charger when I'm on the road for opportunity charging.

 

[vax]

What do you think of this:
http://www.batterystuff.com/battery-chargers/12-volt/multi-volt-input/BT4b022-0148.html
It's a bit weakfor 17Ah batteries?

 

[fechter]

What do you think of this:
http://www.batterystuff.com/battery-chargers/12-volt/multi-volt-input/BT4b022-0148.html
It's a bit weakfor 17Ah batteries?

Some bank chargers won't work with batteries that are in series (without first disconnecting them).
Their prices seem pretty high too. 2amps would be OK if you're not in a hurry to charge.

You could just get 4 of these:
http://shop.oreillyauto.com/ProductDetail.aspx?MfrCode=PNE&MfrPartNumber=610004&CategoryCode=3014 or something similar.

To charge batteries in series, the charger output needs to be isolated from the AC ground pin or you'll get a short when you connect more than one. Some chargers are isolated and some are not. You can measure resistance (ohms) from each charger output wire to the ground pin on the AC plug to see if there is a connection. If the charger has a 2 prong AC plug, you don't need to worry.

 

[likes2bike]

Here is a pic of Manzanita Micro's BMS boards mounted on a backplane ready to install behind the backseat of an EV I work on. If you are not buying into his charger too, to take advantage of the interface, you lose some functionality, but these boards work standalone. there are LED's for lovo and bypassing, with adjustable voltage sttings

 

[africanandy187]

When i finally get my kit and get it installed (any time this week wooohoo!) i will be building a LM741 opamp voltage comparator/low and high voltage indicator BMS system for my lead acid pack. SLA is all i can afford so i have to make the most out of it! I have 3 x 12v 14AH SLA's. The BMS system i intend to make will be a high and low voltage detector on each battery.

When it is high voltage, > ~13.6v It will light up an green LED and i will have a transistor switch a small load(maybe a lightbulb or a 5w resistor), so this will act as a balancer on each battery. Then when it is low voltage perhaps < ~ 12v not under load (i will need to find the value of the battery when the motor just starts to turn under load, probably about 11v?) it will light up a red LED and a transistor will switch a normally closed relay to open and cut the battery out of the pack

That zener bulb configuration works fine, the problem is the zener voltages vary so much that it defeats the object of trying to balance the pack perfectly

To get the most life out of the batteries i don't want to discharge them too much, 70 DOD max (about 12v), i was just wondering what other people run their batteries down to?

 

[TIBOR]

Hi everyone! Without the intention to hijack Vax' topics, please let me ask you guys a few question, somehow similar with the problem risen here, about charging SLA batteries connected in series, separately.
A couple of weeks ago, my 36V/1900mA smart charger for SLA's went out of order. An electronic specialist told me, that the charger can't be repaired. So I decided to buy a new charger, but this time I want to go the way, to charge the batteries separately. As a bike mechanic without any knowlege in electronics, charging with a 36V charger was clear, but charging three separate 12V batteries connected in series, it seems to me a little bit different, so I need some help.
First of all, I would like to charge the three batteries together, in the same time but, if is possible without disconnecting the series connection of the batteries. Yes I read what Fechter wrote about this:

"To charge batteries in series, the charger output needs to be isolated from the AC ground pin or you'll get a short when you connect more than one. Some chargers are isolated and some are not. You can measure resistance (ohms) from each charger output wire to the ground pin on the AC plug to see if there is a connection. If the charger has a 2 prong AC plug, you don't need to worry."

But as I said, very beginner in electronics, can somebody tell me what it is a 2 prong AC plug? How can I know, if my chargers have 2 prong AC plugs? (I bought "no name" 12V/30W three steps smart chargers, just what I found in my town, probably made in China, labeled in Poland and sold in Romania, without any users information.)
If finally I can't go this way, please let me know any other possibility to recharge three batteries in the same time, separately, as simple as possible. I carry all the time with me the charger -now the chargers- above the batteries, for any charging opportunity, and I would like to avoid time-consuming, complicated manoeuvers every time to connect the batteries to the chargers. With the old 36V charger it took me two short cliks and the batteries were in charging state.
Thanks, and I only hope that this old topic will get from the fifteenth page to the first page again, and my questions will be aswered.

 

[Lock]

Hi Tibor... One simple approach is to wire the batteries in both series (for discharge) with a switch and more wires that reconfigures the pack as all batts in parallel... so just one 12V charger needed... Slower to recharge of course, unless you use a higher Amp charger...
Cheers
Lock

 

[amberwolf]

Well, Tibor, first, you really should start your own topic instead of hijacking this one, because it's not the same question, really.

But to answer your prongs question, it refers to the part of the cord you plug into the wall. In some countries/regions, there are two prongs (hot and neutral) for non-grounded AC sockets at the wall outlet, and three prongs (hot, neutral and ground) for grounded AC sockets.

Other places may not use the same standard, so your plugs may be different. It's even possible that you live in a place where it is required that all sockets/plugs include a ground. If you are not sure, you should ask a local electrician to be sure, although there are places on the web such as Wikipedia that do have articles describing many, perhaps most, of the world's differing electrical standards and practices.

 

[vanilla ice]

I would (do) use individual 12v chargers. They are pretty cheap! Maybe ten to fifteen a piece at a good deal. And its not so complex for the voltages we use for bikes. What 3 to 5 chargers, big deal. Sucks that you can't balance down to the cell level though. I'd love to buy some affordable 8-16ah cylindrical single cell 2v agm cells! Pipe dream..

 

[heathyoung]

I'd be suggesting using comparators as comparators - 741's are a pretty dated component, use something like a LM393 - much more suited to the task.

When i finally get my kit and get it installed (any time this week wooohoo!) i will be building a LM741 opamp voltage comparator/low and high voltage indicator BMS system for my lead acid pack. SLA is all i can afford so i have to make the most out of it! I have 3 x 12v 14AH SLA's. The BMS system i intend to make will be a high and low voltage detector on each battery.

When it is high voltage, > ~13.6v It will light up an green LED and i will have a transistor switch a small load(maybe a lightbulb or a 5w resistor), so this will act as a balancer on each battery. Then when it is low voltage perhaps < ~ 12v not under load (i will need to find the value of the battery when the motor just starts to turn under load, probably about 11v?) it will light up a red LED and a transistor will switch a normally closed relay to open and cut the battery out of the pack

That zener bulb configuration works fine, the problem is the zener voltages vary so much that it defeats the object of trying to balance the pack perfectly

To get the most life out of the batteries i don't want to discharge them too much, 70 DOD max (about 12v), i was just wondering what other people run their batteries down to?

 

[TIBOR]

Thanks for everyone. Especially for the lesson about the "prongs". Because my chargers have cords with two prong AC plug for the 220V wall socket and three prong plug???? for the socket in the charger, I think I give up the way to charge the batteries simultaneously without disconnecting them from the series connection. As Lock suggested, I will put a switch between the batteries and I will interrupt the series connection before starting to charge the batteries.
About using comparators, I think I need to learn a little bit more about electronics.
Any way thanks again for all of you.
Tibor