hooking batteries in paralell

[bentendo64]

what would happen if i hooked 2 batteries of different mAh in paralell?
would the larger capacity have to 'charge' the lower capacity battery all the time?
or, would the capacities add up to make a larger one.
I've been thinking of this for a while, and cant come up with a solution

[timmeh87]

Im pretty sure its ok... What kind of battery is it?

[OMGpedobear]

most batteries can be hooked up in parralel , lion are a little but fussy(more so than lipo) but as long as you are careful they should be ok. obviusly the lower capcity abttery will ruyn out first, but if you use nicad this shiouldnt be a problem. Oh and linux is awesome, i should know. and its completely free and my time is precious.

[timmeh87]

The lower capacity battery wont run out first if they are in parallel... They cant be at different voltages.

[superdeformed]

If you hook up two batteries in parallel, depending on the battery type and how well the cells are matched (impedance wise), you may run into the issue of the batteries slowly discharging themselves (faster than they would if just left sitting). Also, if one of the batteries has a lower capacity than the other, that battery will hit the knee voltage on its discharge curve (the point where the voltage starts to rapidly drop off) before the other. When this happens, the higher capacity battery will start to discharge through the lower capacity one to maintain the same voltage between the two. Depending on how sharply the voltage of the lower capacity battery drops off this can be a pretty big current since it will only be limited by the wiring and internal resistance of the batteries.

Overall, you'll end up with a capacity a little higher than twice the capacity of the smaller one. So, while in theory it will still work, you have to be careful of high currents flowing between the two batteries and would be better off using two batteries of the same capacity made with matched cells.

[Rekarp]

If you hook up two batteries in parallel, depending on the battery type and how well the cells are matched (impedance wise), you may run into the issue of the batteries slowly discharging themselves (faster than they would if just left sitting). Also, if one of the batteries has a lower capacity than the other, that battery will hit the knee voltage on its discharge curve (the point where the voltage starts to rapidly drop off) before the other. When this happens, the higher capacity battery will start to discharge through the lower capacity one to maintain the same voltage between the two. Depending on how sharply the voltage of the lower capacity battery drops off this can be a pretty big current since it will only be limited by the wiring and internal resistance of the batteries.

Overall, you'll end up with a capacity a little higher than twice the capacity of the smaller one. So, while in theory it will still work, you have to be careful of high currents flowing between the two batteries and would be better off using two batteries of the same capacity made with matched cells.

Otherwords you can be ok ooooooooooooooooooooorrr it can explode in your lap.

[timmeh87]

If they are two completely different batteries, I dont see why they would have to discharge at the same rate... The above discussion is making that assumption. The smaller one could discharge slower or faster... depending on the cell charge level and impedance.

However, if the discharge curve is sufficiently steep enough, I postulate that the smaller one would discharge at a rate nearly proportional to their difference in size. If it were to discharge any faster, its voltage would go down a little, and it would contribute less to the total current.

What exactly is the logic behind saying the capacity would be limited to twice that of the small one. Where does the extra power go?

[superdeformed]

I made the assumption that they would discharge at more or less the same rate thinking you would need a pretty big difference in voltage or impedance between the two batteries otherwise, though thinking about it more after you pointed it out, the difference in impedance could have a pretty big effect on this. Your argument on the smaller one discharging at a slower rate makes sense, but I think that before you hit the limit of the discharge curves the current from each battery will depend mostly on its impedance and the voltages between the two batteries will stay pretty close (since when one goes down enough, the other will dump charge into it until they are close to equal again).

As for my reasoning on the capacity, I figured that when you've used up the effective capacity of the smaller battery, the larger one will be constantly dumping current in to it, so you'd be wasting charge (at useful voltages) in trying to maintain equal voltages between the batteries. The overall capacity will still be higher than twice the smallest capacity, but it won't be as high as the sum of the two different capacities (assuming that the minimum voltage you need is somewhere near the normal battery voltage).

Of course, this is all just theorizing based on my experience with batteries (mostly NiMH and NiCd), which generally does not involve putting them in parallel, so if anyone has other ideas or has actually tried this I'd be happy to hear it . Also, all of the voltage balancing concerns could be avoided by just putting a diode on the output of each battery if the voltage drop won't cause any operating issues.

[timmeh87]

As for my reasoning on the capacity, I figured that when you've used up the effective capacity of the smaller battery, the larger one will be constantly dumping current in to it, so you'd be wasting charge (at useful voltages) in trying to maintain equal voltages between the batteries. The overall capacity will still be higher than twice the smallest capacity, but it won't be as high as the sum of the two different capacities (assuming that the minimum voltage you need is somewhere near the normal battery voltage).

BUT. If the large battery charges the small battery... then the small battery would be a little bit charged. Wouldn't it then be able to power the circuit a little more?

[Rekarp]

As for my reasoning on the capacity, I figured that when you've used up the effective capacity of the smaller battery, the larger one will be constantly dumping current in to it, so you'd be wasting charge (at useful voltages) in trying to maintain equal voltages between the batteries. The overall capacity will still be higher than twice the smallest capacity, but it won't be as high as the sum of the two different capacities (assuming that the minimum voltage you need is somewhere near the normal battery voltage).

BUT. If the large battery charges the small battery... then the small battery would be a little bit charged. Wouldn't it then be able to power the circuit a little more?

your going to lose allot of power when you start charging the small battery. Charging batteries is no where near 80% efficient.

[timmeh87]

Based on what math?

[Rekarp]

tis what my professor said but looking at spec sheets its close to 99% so he may be talking about lead acid...

Kids sleeping in class is not good mmk?