I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, here.
"In theory", as all of the batteries see the same load, they should all degrade in lock step.
"In practice", you expect some variations.
How much can I potentially gain (life, capacity, etc.) if I design a "stacked charger" (e.g., 4 floating 12V chargers) instead of a single 48V charger?
At the very least, it should facilitate replacing single batteries (instead of replacing the entire stack at the same time) -- ?
not a whole lot. if you really want to balance them you should be using
2V cells. you'd need 24, and 24 chargers.-- This email has not been checked by half-arsed antivirus software
If you want imbalance, replace single batteries.
If you want balance, simplest is to build a voltage divider and hook it to the batteries. Set them at 1% of the load and don't worry about it. I pulled the 1% out of my ass. Depends on charge current load current, duty factor...pick a number.
This assumes they're charged frequently. If not, put the load on a connector that you can plug in when you charge 'em.
For batteries that are disconnected when charged, charge them in parallel, with some current limiting light bulbs between 'em. I did that with my bicycle light. 4 lithiums all brought out to a connector. The bike runs them in series. Charged in parallel, again with some current limits so they don't blow up if mismatched.
Eons ago I designed a chip to charge-control multi-cell LiIon batteries. It used shunting MOS FET's across each cell and a MUX'd sensing system to "balance" the pack. Customer was California Micro Devices (Tempe, near MicroRel, also one of my customers ;-). I'm not sure if they even still exist. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 |
Ha! Interesting approach -- tweek the load to make one charger "see" it as "consistent".
I'd think it would be relatively easy to hack together a set of identical, floated 12V chargers (or 6V if I changed to a stack of 8).
I'm just not sure how much benefit it will yield.
Looks like CMD was absorbed by ON Semi:
...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 |
The devil is in the details. You didn't say what type of battery. Makes a HUGE difference.
-- Rick C
They are charged while "in use". Think of it as a 48V DC supply like CO battery.
But, I don't have TPC's budget, nor staff! :-/
IIRC the balancing needs of lead-acid cells are less than LiPo, so you can profit by charging them in units of 12V.
-- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
External charge balancers for LiPo batteries just burn current on the higher-voltage cells to keep them in line with the lower-voltage cells. You should be able to do the same thing with your 12V cells.
Google around -- there's balancing circuits out there that you can crib off of.
-- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
I imagine the gain would be too small to be worth it.
If the batteries are exactly the same age and have seen the exact same use, I bet the rest won't be far behind when the first one packs up.
Maybe you can get a bit of discount when you buy four batteries at once in stead one at a time?
-- RoRo
I'm not looking for a "how to" but, rather, trying to understand the likely benefits/consequences of this sort of approach.
Said another way, other than cost, what are the *downsides*?
If, for example, it provides no *practical* benefit, then you've just added cost and complexity for dubious gains.
Imagine starting out with four batteries (A, B, C, D) "freshly purchased".
*Hopefully*, from the same manufacturer and the same lot code. I.e., as close to "identical" as you can get -- without paying for special testing/characterization.You stack them and put them into use.
Over time (based on usage, environmental factors, etc.), the "weakest" battery manfests and starts underperforming. Your fancy charger tries to make the best of things -- preventing its altered characteristics from cooking other batteries in the stack.
Eventually, you replace that battery -- with a fresh *new* battery. You've effectively deferred replacing three of the batteries.
Now, the other three batteries "look weak", in comparison. But, you can compensate for this, etc.
However, if a *second* battery is likely to fail NEXT WEEK, you haven't really "saved" much. In fact, you've now had to "service" the pack twice, instead of just once.
IN PRACTICE, do you just end up staggering battery replacements and not really saving much money (which would be the case if batteries failed at roughly the same time) *or* time (multiple efforts to rebuild the pack).
Worse, does the constant influx of "fresh" batteries aggravate the situation?
I.e., if battery failures have a large standard deviation (in terms of time) and very little impact on the longevity of their "electrical neighbors", then you can potentially save money by treating each failure as an isolated event. OTOH, if the failures are clustered (in time), then the better approach is simply to discard all of them when *any* shows signs of failure -- the others will likely SOON follow.
Battery management policy (as well as hardware) factors into TCO in a big way. E.g., for me to replace a dozen batteries WHEN THE FIRST ONE STARTS TO FAIL is a few hundred dollars. If the first one is a possible outlier and not a predictor of the balance of the pack, then its silly to let *it* drive your replacement strategy.
No need for the lecture, you're preaching to the choir.
I can't remember where I learned it, but for lead-acid I recall seeing that balancing starts getting worthwhile for anything over 6 cells in series (12V).
-- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
, l . s
Lead acid battery capacity drops greatly over time. If you put a new batter y into an old stack it'll be 2 or 3 times the capacity of the others. That is not going to work so well.
OTOH if you're willing to rearrange a bunch of 48v stacks you might group s tacks of low, medium & high capacity. But labour costs creep up & up then. No... I don't have figures on longevity. Capacity deterioration is one of t he main reasons for me to replace lead acids, I junk them at around 1/3 ori ginal capacity.
There is of course another approach to bad cells. Lots of 2v in series each with some sort of electronic diode that ensures negative voltage can't occ ur on the cell. Now use a stack of >48v, and Bob remains your uncle even wh en several of the cells are dead. No extra labour involved. Or if your app can tolerate 48+6v you could just stack 6v units, giving you continuing 48v service with one failure only. 42v for 2 failures etc.
NT
I think the diode wants to be in parallel with each cell. A diode in series only prevents it from being charged.
When the cell voltage is high a parallel diode is reverse biased. When cell voltage reverses because the other cells are still forcing current through a dead cell the diode conducts and the reverse voltage the cells sees is limited to the forward voltage of the diode. I guess that is better than nothing, but still not so good.
I think a practical circuit would need to have an active monitoring of the cell voltage and switch out the cell and switch in a bypass transistor to prevent reversing the cell voltage.
-- Rick C
I assumed the OP could work that out!
you mean like a electronic diode with zero Vf? :) I'll get you a coffee.
NT
Not sure what the coffee is for. The Vf is not the issue. The issue is to take the cell out of the circuit *before* it is down to zero volts. The diode circuit won't kick in until *after* the cell is reverse polarity by the Vf. Better if the cell is taken out of the circuit before it gets to that point.
-- Rick C
Whether the diode circuit conducts a bit before 0v is reached or at 0v is fairly immaterial imho. When a cell is down to 0.1 or 0.2v it won't pass much current through a short. Waiting for it to hit 0v is a very trivial gain.
The coffee was because you suggested exactly what I just had, a diode circuit that did not take the cell to reverse volts, ie has a Vf of zero.
NT
We don't seem to be talking the same language. The diode circuit does not kick in until the cell is already at reverse voltage enough to forward bias the diode. I don't know why you keep talking about 0 volts. So the diode circuit won't just drain the cell to zero volts, it will apply a *reverse* voltage to the cell.
The smart switch can kick in at any voltage desired. This is not a diode circuit at all. It is a transistor controlled by a circuit or program that detects that the cell voltage is low enough that it is switched *out* of the circuit and the cell bypassed. I don't know of any way you can do this with diodes alone. This circuit does not emulate a diode in any way.
-- Rick C
Obviously one designs the diode circuit to short at 0v, not negative cell voltage. Unless you opt to be cheap and just use a single shottky diode, which would be beter than nothing, but not ideal.
It does the same thing a diode does, except that Vf=0.
NT
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