Charging times for Nicads and NiMh

[Kit Carson]

  Can some kind soul help me, How do you work out charging time for Nimh and NiCad batteries. My charger output current is 120mA. Kit

[Ian Moore]

Hi Kit

 

if you have a fixed output from your charger, then you just divide the battery capacity in mAHr ie a normal rx battery may be 1000mA, well that will be 1000 divided by 120 which will give you a charge time of approx 8.3hrs. and a charge rate just over the C/10 rate for trickle charging these type of batteries, ie it is always best to try and charge batteries at one tenth their capacity as to not damage them. at c/10 charge rate the batteries can be left on charge as long as you want without harming them.

 

Hope this helps

 

Ian

[Tim Mackey]

Well I am sorry to disagree here, but disagree I must.

Although most of what Ian says ( incidentally "hello Ian" ) is about right, I would not recommend slow charging ( .05 - .1C rate ) as the normal method for nickel metal hydride cells. This is fine to use for the initial forming charge of a new pack, and should be at the rate calculated above  X 1.2 as there will be inefficiencies in any charge cycle, and you ideally need to pump in around 120% of stated capacity to ensure fully forming. Try this alternative calculation method for the first forming charge. If your battery is 1000mAh and your charger is fixed at 120mA then (1000/120) X1.2 = 10 hours.

Normally, forming, or slow charging of NiCD is done at .1C and NiMHs even lower at around .05C which in your battery's case would be 50 mA.

However NiMH cells are NOT best charged at 1/10C and leaving them on charge at this rate for as long as you like will harm them.  NiMH do NOT tolerate slow charging well at all, and will likely overheat and get pushed past their designed maximum charge level of around 1.6V each cell. The only sensible and safe way to charge them is with a proper NiMH capable smart charger, at around .5 - 1C rate ( in your case 500 - 1000mA ) and let the charger use either temperature sensing ( if so equipped ) or NDV ( negative delta voltage ) peak detect systems or both, to correctly determine when to stop charging. NDV is notoriously difficult to implement at charge rates below .5C. Leaving your packs connected to a 100mA charger for more than around 10 -12 hours maximum WILL damage them. Even though I stated in my recent magazine article that most NiCD chargers will happily accomodate NiMH cells, in an ideal world you should really use a dedicated NiMH charge algorithm, and remember,  trickle charge rates for NiMH should actually be lower than those of NiCD at around .05C.

Slow charging NiMH cells causes crystallisation on the plates, and leads to increased "memory" effect and shorter life.

[Bing]

At what rate would you recommend cycling a 1500 nimh battery as when I try to cycle my trannie battery at 2 amps it blows the fuse.

[Bruce Richards]

1-1.5A charge rate and 300-500ma discharge would be what I would do.

[Bing]

Cheers Bruce I'll give it a go and let you know.

[Bing]

Hi Bruce, I cycled at your suggested rate and it worked although battery got quite warm. Thanks for your help.

[Bruce Richards]

Bing warm is OK but hot is not. If you thin k they are getting to hot, in future go for a lower charge current. If the charge current is too low then the charger may not detect when the battery is full.

[Bing]

OK Bruce I'll give that a go, thanks