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[safe]

I am truly confused. Since when did cell balancing apply to SLA Battery Packs ?

Normally it doesn't as people just figure they will not bother to get the full life out of them. But if someone were to want to extend the life of a set of SLA's it still actually makes some sense if the PRICE is low enough.

I charge in parallel, so charge balancing is not an issue, but when in runtime the differences between cell capacity means that one cell will always be the weakest and wear out the fastest. (because it will end at a lower voltage than the others every time)

SLA is cheap... after a thousand miles the easiest thing is just to replace the weakest cell, but if you had balanced as you rode then you might get to two thousand before you would replace all the cells at once.

The difference is really one of "replace all at once" or "replace one at a time". But when you do it one at a time (allowing the "runt" to die a slow death) you actually get fewer total Watt Hours out of the cell compared to balancing.

It's totally different than something like NiCads that don't wear out at all by having low voltage... you have to remember that SLA is a lot like LiFePO4 in that low voltage really wears them out.

SLA could use non-resistive runtime balancing... but is it cost effective?

 

[Ypedal]

.... you have to remember that SLA is a lot like LiFePO4 in that low voltage really wears them out.

Absolutely not true...

 

[Battboy]

Sorry, Safe, but I do not see anyway to change individual Cell Plates inside of an SLA battery. It just isn't done - even by the people who manufacture this chemistry. SLA or Lead Acid has one big problem that leads to the batteries not lasting as long as they have been manufactured to last - and that big problem is called "sulphation". Look it up. There are several ways to help slow down sulphation, but none that can completely eliminate it. Once a battery is too far sulphanated it's generally a lost cause and has to be replaced. It just can't deliver electrons anymore due to the sulfur crystals blocking the transmission.

So, you theory of Cell Balancing a Lead Acid Pack just doesn't have any validity at all and whatever you think you can do to change this basic chemical fact just isn't going to work like you think it will.

Best Regards,

Don Harmon

 

[safe]

Absolutely not true...

You've got to be kidding... low voltage (fully drained) SLA cells don't get damage?

Are you kidding?

 

[Ypedal]

Absolutely not true...

You've got to be kidding... low voltage (fully drained) SLA cells don't get damage?

Are you kidding?

Out of context.. you said " Low voltage "..

An sla left at 11v resting voltage will be useless in a short period of time..

A LiFePo4 cell left at 2.5v(no charge left, or very little ) will be just fine for months..

 

[safe]

Sorry, Safe, but I do not see anyway to change individual Cell Plates inside of an SLA battery.

No, I'm not talking about tinkering with the sub-cells within a 12 volt battery.

If you are running a 48 volt system then you have four 12 volt cells in series and that means that one of those cells will be slightly lower in capacity than others. Over time that difference grows and grows until that "runt" cell has essentially died out. The other three are fine, but have some level of lowered capacity. (they might be at 70% when the "runt" is now down to 40%)

So the idea would be to drain all the cells equally and balance them as you rode so that over time they all wear out equally.

Larger EV's balance their SLA cells, so it's not like no one does this, but since our SLA ebikes tend to be the super-cheapo models they tend to not get the attention that the bigger and more expensive one's get.

The reason people want to balance their LiFePO4's is that they are so expensive that the idea of allowing premature aging would be too expensive and seriously hurt the long term cost effectiveness.

The Chevy Volt concept was to keep everything in balance between a range of 20%-80% for the same reasons.

The dynamic tension is:

Cheap SLA eBike <--------> Expensive Balancing Circuits

...that's why people aren't doing it.

Something like a four cell switched capacitor solution that only operated at runtime would be perfect.

Home built would be good.

Simple is best.

 

[nomad85]

The reason people want to balance their LiFePO4's is that they are so expensive that the idea of allowing premature aging would be too expensive and seriously hurt the long term cost effectiveness.

I'm no expert, but I do use LiFePO4 on my bike and Lipo in my RC plane. I use a balancer because I have to, not because the batteries are more expensive. My rc lipo packs are only 20$ but they need to be monitored during the charge to ensure proper function. I don't use balancer on my SLA, because I don't need to. I'm pretty sure that SLA can handle some overcharging, whereas Lithium cannot. I think it has more to do with the way the cells work and what they need as apposed to their cost.

 

[safe]

Existing Product

PowerCheq Active Battery Equalizer Module

http://www.evsource.com/tls_powercheq.php

The only problem is that the price is $58.99 per cell, which for me would mean $235.96 and that is totally insane for an ebike with $100 worth of batteries.

But they do have products for SLA... you do need RUNTIME balancing if you want to extend the lifetime of the cells.

http://powerdesigners.com/powercheq_faq.htm

http://www.evdeals.com/PowerCheq.htm

http://www.evdeals.com/images/Parts/Image5.gif

I just want to get this kind of functionality without the nasty price... :lol:

 

[Battboy]

If you don't want to pay the price - then just charge your batteries individually. At least do that every once in awhile and you will probably get similar results ?

Don Harmon

 

[voicecoils]

Existing Product

The only problem is that the price is $58.99 per cell, which for me would mean $235.96 and that is totally insane for an ebike with $100 worth of batteries.

I just want to get this kind of functionality without the nasty price... :lol:

As I wrote in reply to your post before, you need 3 powercheq's for FOUR batteries. So your price is incorrect.

I know you are discounting the possibility of using them immediately based on price, but you should study them carefully to understand what they actually do, what they are designed for, and when they make economic sense in the larger picture of the powersystem.

Perhaps it would be best for you to read the FAQ: http://www.powerdesigners.com/powercheq_faq.htm

You want equlisation during discharge, not charge as I understand since you charge with single cells. Read the tech specs more closely then to understand what the limitations of the current product is. As I stated before there are very few options for SLAs and none of them are priced like a pack of chewing gum.

The following quote from an Aussie flick applies here "Tell 'em they're dreamin'!"

 

[ZapPat]

I think that just a low voltage cutoff on each SLA battery would be all you need to greatly extend your weakest batterie's life. I understand the problem, since I damaged one of my fairly new and very expensive enersys odyssey batteries recently from over discharge of the 36V string. This one battery now seems to have a damaged cell inside, which could probably have been avoided if I had some form of LVC for each battery.

I think that lead batteries would last much longer than they do if they had a LVC per cell, and this circuit could be re-used when changing the battery to reduce costs (but this won't happen since LiFePO4 is now very competitive costwise). As for balancing, lead is supposedly quite tolerant of a fair amount of overcharge current without damage, which explains why they even last any significant amount of time without a BMS.

Of course safe's idea of charge balancing during discharge would be great, but I think it is just too costly a solution for lead. The simpler LVC per battery would protect the weakest batt from getting much worse over time. Besides, for discharge balancing to be effective, the balancing system would have to be able to shuttle reletively high currents to the weakest battery.

 

[safe]

...just charge your batteries individually. At least do that every once in awhile and you will probably get similar results?

I charge my cells in parallel and run them in series. I've tried charging them individually, but it makes no difference because even the weak cell has the same full charge voltage.

All cells (strong and weak) tend to fill to the same voltage.

Strong cells deliver more capacity than weak cells.

So as the cells in series release their energy in runtime discharge the weaker cell always runs out first.

People need to realize that ANY power usage with SLA's causes some damage because SLA prefers to be kept fully charged all the time. So cutting it off at some predetermined level doesn't really change the situation it just decides how much damage you are willing to accept for that ride.

A good balancer needs to actually transfer energy and not dissipate it.

You want equlisation during discharge, not charge as I understand since you charge with single cells. Read the tech specs more closely then to understand what the limitations of the current product is. As I stated before there are very few options for SLAs and none of them are priced like a pack of chewing gum.

That's why for low priced ebikes using SLA the real issue is price. If your batteries only cost $100 it doesn't make much sense to have to buy a balancer that costs $200. I want a balancer that IS priced like a pack of chewing gum (funny line :lol: ) or at least around $20 to make any sense in doing it.

 

[methods]

Then build it from harvested parts found via dumpster diving

One only needs to look at the 24V BMS thread to realize that the parts alone are $80 for that *simple* 24 channel circuit that can only shuttle around 500ma. Under load you would need to shuttle a lot more current than that to keep things balanced the way you describe.

When the rubber hits the road, the only way you are going to get chewing gum prices is to source everything for free. Design the circuit using a pen and paper, harvest the parts from dumpsters, solder them together on a perf board, test, and redesign.

Not even the Chinese will sell you a device for less than the sum total wholesale cost of the parts within.

The copper wire required will cost more than a pack of chewing gum. . .

Apply the "cost of parts" logic to your argument and perhaps you can understand why you will not find what you seek.

-methods

 

[safe]

You are probably right... a $20 four cell SLA cell balancer is probably not realistic...

 

[safe]

Just out of curiosity... what parts would be needed to build something like a four cell switched capacitor balancer. To make things easier the circuit could be designed to only operate when the bike is running (when the cells are connected) so that one can ignore issues of draining the cells when the bike is at rest.

Think of four SLA cells.

Charge them in parallel. (no problems here, balancer not connected)

Run the cells in series and when you make the series connection the switched capacitor balancer turns on.

...with such low expectations of refinement this seems to be just a matter of buying some capacitors.

 

[cerewa]

What component is used to switch the capacitors?