Charging Ni-MH AA Batteries - for how long?

[alldownwind]

I've got an Icom M3-Euro handheld VHF (no longer made, I think, but an excellent set).
It runs on 6 x AA cells in series, and you can use disposables or rechargeables, i.e. 9v or 7.2v (there's a switch you set depending on which you are using).
The battery charger supplied puts out 12v at 500mA.
The set came with 6 x 700mAh rechargeables, and the instructions said to charge for 14 hours.
The question is this:
I have just replaced the batteries, I now have 6 x 2000mA Ni-MH cells (£1 each from Maplin, special offer on at the moment). Is there a battery expert out there who can tell me how long a full charge will take, please?

 

[FrankieJ]

I'm no battery expert but I know that the higher the M.A. the longer they last ,so I try to buy the biggest capacity available.My last purchase was AA size 2800 m.a. from Sevendayshop. com. along with a quick charger that runs off a 12v supply . It is automatic, so I don't have to worry about how long to recharge.
I know I haven't answered your question but it may help.

 

[HenryB]

Your original charging instructions doesn't seem right to me - if the charger outputs 500 mA then charging a 700 mAh battery from flat shouldn't take much longer than 1.4 hours, definitely not 14 hours.
As to your new batteries - if in doubt charge them until they begin to feel warm. As I understand it, the batteries won't significantly increase in temperature while they are taking a charge but will once fully charged. Automatic chargers often sense the temperature to decide when to stop charging.

 

[BenchHead]

I have a UniRoss charger - its says 285-320mA - it isn't automatic but the instructions say approx 6 hours for 1300mAh AAs and approx 9 hours for 2000mAh AAs.

 

[Spyro]

From The instructions on my charger it gives a formula which is battery capacity in mAhr divided by charge current times 1.4 gives you the answer in hours.
in my case 2400/120 X 1.4 = 28 so charge time is 28hrs.
Don't know where the 1.4 comes from.

This seems a while but the output capacity is low compared to yours.
Maybe you have to divide the output capacity by the number of batteries you are charging?
The output is 1.2 volts you said 12V is that a mistake

 

[William_H]

Because the charger is rated at 500 ma doesn't mean that it will charge at 500 ma. It needs to be a higher rating for safety in case of fully flat batteries or shorted batteries (often a failure mode)

In fact we might guess that if 700ma H batteries take 14 hrs then the current will be 70ma.
14 hrs is a notional time for charging at 10 hr rate (extra 4 hrs for loss)
This rate is the notional figure for the max current that can be safely charged into the batteries for very long term without damage.(without any fancy full charge sensing circuit) Hence it is or was the common full charge time. (batteries and chargers are often these days designed for fast charge with a fully charged sensing circuit of some sort)

My guess then is that your charger is simply resistive current limiting from the 12 volts out of the transformer giving 70 ma into the battery. This means that 2000maH batteries will take 35 hrs for a full charge. ie 2000maH at 70 ma plus 40%.

This means that your batteries will be even safer with long term connection to the charger but will take longer for recharge from flat. (as supplied) I would suggest that you will find the old charger fine for the job with the new bigger batteries. olewill

 

[AngusMcDoon]

When I bought some NiMH AAs from Maplin, I got a charger too. It takes 4 batteries, runs off mains or 12V, came with a 12V car connector, is fully automatic with a indicator showing what it's doing, and takes 40 mins to 1 hour to do a full charge. Charges AAAs as well. All for about £13.

 

[gandy]

We have an M3 as well. Ours came with existing NiCad batteries, and I would expect that the Icom charger would suit NiCad rather than Ni-MH. If its putting out 500mA its going to kill your batteries unless its smart enough to switch off when they're full.

We don't have the mains charger at all, I take the batteries out and charge them in a general-purpose AA battery charger.

 

[gjgm]

Nicad and Nimh have different peak charge and discharge behaviour. You can fast charge them with appropriate fast chargers.. take a look at hobby stores for model cars and model planes, overlander, alshobbies,brchobbies,ripmax,jpperkins..
Problem with rechargeables is they discharge while left standing, so you really do need to keep on top of their useful state. But you can fast charge them in minutes...

 

[alldownwind]

Thanks all for this.
I've now remembered from my R/C model flying days that Nicads were always charged at a rate equal to 1/10th of the capacity, for 14hrs, as olewill has said in another way.
Perhaps the best bet is indeed to use a dedicated AA charger (got one of those as well) altho slightly inconvenient as the radio uses 6 cells and my charger takes 4.
Thanks again everyone.

 

[Achilles]

I have the same handheld and wanted to change the Ni-Cad batteries for NI-mh because of the memory affect with the Ni-Cads. You can use Ni-Mh in the M3 but you mustn't charge them with the charger that came with the M3, ICOM told me this.

 

[Drascomber]

I have a Raymarine (101E I think) which is great but like the M3 will take 6 AA NiMH's. Anyone know where you can get an intelligent fast charger that will take up to 6 AA's?

 

[VicMallows]

Well explained as always olewill /forums/images/graemlins/smile.gif

I find that with the various capacities of batteries these days and the (generally unknown) characteristics of chargers (whether mains or 12v types), the only reliable method of working out the charging time in any situation is to actually measure the charge current. Best to monitor after about an hour when it has stabilsed from the initial inrush.

(before anyone asks, the easiest way is to equip youreself with a selection of the common power connectors (Maplin) and make up some suitable adapters).

Vic