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Topic: A thought about charging batteries (Read 2630 times)previous topic - next topic

scswift

Jan 12, 2011, 04:33 pmLast Edit: Jan 12, 2011, 04:48 pm by scswift Reason: 1
I was just reading an article on electric cars, and got to thinking about how resistance drops when you wire resistors in parrallel.

Now I'm still a noob to electronics, so I have no idea if this idea has any merit, but the thought I just had now was this:

When you charge a battery, like say, one of those RC batteries... it gets really hot.  And heat is generated when there is a lot of resistance and you're trying to put a lot of current through at once.  Batteries can explode from this heat, which I suppose, along with breakdown of the chemicals, may be one of the limiting factors in charging them fast.

So I got to thinking... Those batteries in the nicad packs are wired in series.  The current has to flow through them all.  If the batteries have resistance, that will add up.  But what if the batteries were wired in parralel?  Just for the charge cycle.  Wouldn't that be kind of like wiring resistors in parallel?  And wouldn't that lower the overall resistance to the current?  Might that keep the batteries cooler, because battery #1 doesn't have to pass all the juice for 6 batteries through it?  And thus, wouldn't that allow you to charge the batteires faster without breaking down the chemicals or risking an explosion?

I don't know.  What do you think?

amacmullen14

#1
Jan 12, 2011, 06:17 pm
You's have to use more circuitry, because in parallel, there is no guarantee each battery will get the same amount of current.

Worst case, one battery will receive the current supplied to charge six, or whatever number.  Boom

retrolefty

#2
Jan 12, 2011, 06:21 pm
Quote
I don't know.  What do you think?

I think that battery cell heat will be the same no matter if charged in series or parallel. Charge rate is a trade-off between how long you want to wait for a full charge to accumulate Vs how high a charge current you can force through a cell before damaging it. So for equal charge rate you get equal cell heat, equal charging time, no matter what the battery cell configuration is.

Lefty

AlphaZeta

#3
Jan 12, 2011, 07:31 pm
The only real solutions are to

1. Reduce battery's internal resistance and increase battery efficiency. Unfortunately, this largely depends upon the technology used building the battery and we don't have much control over.

2. Reduce the charging current, which will make the charging time even longer.

The problem with charging batteries in parallel is that it is a lot more complicated than charging them in series. Special care must be taken care of to ensure current distributes evenly among all the parallel connected batteries and the charging current will be much too high for normal circuitry to handle.

And as retrolefty said, the heat generated is only dependent on the current at which the battery is being charged. So why add unnecessary complexity?

Ran Talbott

#4
Jan 12, 2011, 07:44 pm
RC batteries mostly get hot because people are impatient,  and are unwilling to use lower-rate chargers.

Charging cells in series is a real problem.  How severe it is depends on how closely the cells are matched:  if they're not very close,  some will wind up either under- or over-charged.  Over time,  the problem can get worse,  as the mismatched cells get more damaged by heat during fast charging (the "stronger" cells have to dissipate the extra energy as heat while the weak ones are getting topped off) or heavy load (the weaker cells have to work harder). Both the packs that came with my Ryobi tool kit have this problem:  after a couple of years of moderate home use,  the charger started refusing to even try to charge them,  because one or more cells are shot.  And the cost of getting them repaired (which has to be done by a third party,  because Ryobi won't do it) is higher than replacing them

Separate charging of the individual cells is definitely much better,  but the vast majority of people,   especially in "First World" countries,  strongly prefer the convenience of battery packs that are easy to swap quickly,  even though the life-cycle costs are somewhat higher.

There's quite a bit of literature about this available on the web,  if you want to learn the intimate details.

retrolefty

#5
Jan 12, 2011, 08:15 pmLast Edit: Jan 12, 2011, 08:16 pm by retrolefty Reason: 1
Quote
Charging cells in series is a real problem.

That is very true. And people who are serious about their R/C hobby and make large \$\$ investments in expensive Li-Po batteries, buy the batteries that come with 'balance charger' connectors and only use balance chargers that ensure that not only is the battery fully charged at the recommended charge rate for that specific battery, but that also each cell is monitored and can re-balance the pack by bypassing current around individual series cells when fully charged, so that all cells are 100% when done. Li-Po batteries need very special attention in both their use and recharging or else fire and flames will result.

Lefty

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