What's the best way to charge batteries (Nicad or Nimh)? Series or parallel? I'm guessing parallel, but as I have a rechargeable light which charges the battery in series is it worth taking them out of the light to charge them with an external charger in parallel?
Cheers
Steve
It must depend on the voltage of the charger. 2 batteries in series would need twice the voltage they would need if in parallel.
Rob Graham
Do you mean batteries, or cells?
If you want to e.g. charge a pack of identical cells in more or less identical state of charge, in a hurry, you need a delta peak charger. And they need to be charged in series really.
No, not really. Its fine to charge more or less identical cells in series at the same current. It sonds like its a trickle charge anyway. Eventually the cells that charge first just get warm and dump the excess charge as heat, the others stay on charging till they are full too.
Neither, charge them individually, both series and parallel charging have their problems.
If you charge in parallel there's no way to guarantee that they share the charging current properly, thus the more discharged one will (at least initially) get most of the current and that may be too much. Also it may well be that one cell is old/faulty and won't charge to the full correct voltage so the other (good) cell doesn't get fully charged.
If you charge in series then cells with less capacity (older, more misused) will get overcharged and/or better cells will be undercharged.
When charging battery packs you usually can't avoid charging in series and sets of cells in packs are going to be pretty similar so series charging is acceptable if not ideal.
Well, of course, any pack will be charged in series, so I doubt it makes much difference.
The ideal way would be to charge each cell individually so it can be measured individually as well. But provided the cells aren't run completely flat this shouldn't matter in practice.
That's one way to shorten their service life by a factor of several...
Individually; failing that - in series so long as the size, age and discharge state is similar. Parallel charging won't work with a constant current charger (which all NiMH and NiCd chargers are) and will damage cells.
If you have a power supply (not battery charger) available then provide individual current limit resistors and charge them in parallel - a reasonable charge current is one tenth of capacity.
Dave S
Utterly wrong. How would you ensure equality of current between the cells?
constant voltage source.......
Dave S
Personally I have a conditioning charger that discharges and then charges the cells separatly - takes 4 cells at once. This is to reduce the memory effect of nicads. For devices that have there own chargers I still occasionally remove the cells and put the in the conditioner for a couple of cycles.
Lawrence
usenet at lklyne dt co dt uk
NiCads don't have memory effect (except in the case of satellite applications, with an extremely regular charge/discharge cycle). The term memory effect was stolen from this phenomenon to describe the reducing capacity of over charged, abused cells.
The only use for a discharge is for a timer based charging system (NiCad or NiMh), so you are sure each cell is flat (but not too flat - also damaging) so you don't try and cram a full charge in an already half full cell.
A smart charger that charges each cell individually is the way to go. Costs about £30, and won't fry your batteries like a set time charger.
Also of note, new NiMh cells will get fried if you do a full charge on the first few charges, they take a few cycles to get conditioned. A smart charger will switch off earlier if the cell is ready. I have a friend who took his smart charger back to the shop because of this, and swapped it for a timer based charger because he said it charged them better. If only there were an RSPCA for batteries.
Bob
Unless you limited your charger to an intrinsically safe charge rate of about C/20 this wouldn't work. Alkaline batteries require constant current not constant voltage charging. You could certainly put constant current sources in each battery feed, but this is independent not parallel charging.
Simply putting a resistor from a voltage source would lead to a variable charge rate. If the resistor was sized for the initial charge rate it would be too large for the later charge rate, if sized for the later it would be too small for the initial. The former would have to be the solution chosen but the results would be unsatisfactory.
Not under trickle charge it ain't.
>
And that is *exactly* how most cheap drill chargers work - or did last time I looked. And since they're meant to give a four hour charge - result quickly knackered batteries.
charge them in parallel", meaning that the battery/resistor combinations were in parallel.
The current remains constant providing the supply voltage is high enough that the battery voltage change is insignificant. Let's say that the cell voltage ranges from 0.5 to 1.2V, choose a supply voltage of 15V and a 220R series resistor. Initial charge current = 66mA, final charge current is 63mA. I would have thought this was constant enough to keep a NiCd or NiMH happy. If you want it more constant you simply choose a higher voltage, naturally the power dissipated in the resistor increases with voltage.
Dave S
I don't think so. At least mine appears to be a gebuine delta peak charger.
>
Absolutely, the myth of 'memory effect' in NiCds is blamed for lots of dead NiCds.
The difficulty is to deduce from the advertising hype which chargers really *are* intelligent chargers. Unless they actually mention that they're 'delta V' or similar how is one to tell?
... before anyone complains 'memory effect' isn't totally a myth of course but only occurs in very specific (and thus rare) conditions. It occurs when NiCds are recharged at regular intervals by exactly the same amount. It was noticed (as previous poster said) when NiCds in a satellite were recharged *very* regularly by solar cells. The amount of discharge was always the same and recharging occurred at exactly the same (sunrise) time. After, I believe, some hundreds of cycles like this the NiCds lost capacity.
The chances of repeating this regularity in a domestic situation are miniscule.
I am replying to this message as TNP seems to know a lot about charging batteries.
I have a good quality nicad charger (a Robbe Lader 6+2 if it makes any difference) that I am loath to replace. I was wondering if it was possible to add a delta V switch to the output that could disconnect the cells when they are charged. Does anyone know of a circuit diagram on the web that I could use to add this functionality?
Regards,
Andy.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.