NiMH cells and chargers

[VicS]

To use with my recently purchased camera I bought a "Uniross X Press 700" charger cw a set of 2500mAh AA NiMHs from a well known DIY and home requisites store

I choose that particular charger because it came with a separate mains adapter, rather than being a plug in unit, a 12 volt connector and the recommended NiMHs for the camera. The separate mains adapter makes the charger itself very compact, easily pocket sized.

It should charge the 2500mAh AA cells in 4¼ hours from flat and drop down to a "maintenance" charge rate when they are charged. (- delta V system)

I was surprised that it showed the cells as "ready" after only 20 -30 minutes charging and rather disappointed when they went flat again with very little use.

I have played a bit and discovered that if left for 4 hours they are just a little bit better and if left for 24 hours they are just about fully charged.
If the unit is swtiched off and on again when it shows "ready" it reverts to full charging mode for another period and if this is done repeatedly for four hours or so then they are fully charged.

Has anyone had any similar experiences or have I just been unlucky again and bought a dud one.

Take it back it guess, just a pity I did not buy it at the local store.

I have had a look at what Maplin have to offer. One similar (also includes a USB lead) but "Map" brand and not yet available! Any other suggestions.

 

[ShipsWoofy]

I am no cell expert, but following advice, I always charge mine first time in an older 24hr charger before charging them in the fast chargers I have.

This really does seem to make a difference, a kind of running in period.

I can now fast charge my cells with little noticeable effect on the performance.

An expert will soon be along to tell me I am talking rubbish!

 

[jfkal]

Even NiMH have some memory effect and need occasional conditioning. If your charger is one of the later models it should have a dis-charge function. (Mine has a button you need to press for 5 seconds to start that). The charger takes the battery fully down and cherges up again giving you expected performance again.

 

[malcp]

I have seen similar effects, on a different branded charger and batteries (Powerex batts & Maha charger) when new. After a couple of charge / discharge cycles the system settled down and now works as expected. So, maybe try it a few times before taking it back.
Hope this helps.

 

[gjgm]

how are you assessing what the charge state is?

 

[dedwards]

It could be a duff battery. It looks like that charger is only single channel so rather than assessing the charge state of each battery individually it looks at them as a group. I had a problem like that. Thought the charger was duff. Bought a v good one from maplins which immediately told me that one of the batteries was dead while still charging the other three perfectly.

Get another charger which is able to charge batteries one at a time rather than in pairs and use that as your main charger with the other as a secondary (e.g. on board) charger.

For the time being. try charging them in pairs and see if one pair charges better than the other.

 

[VicS]

Thanks for the comments. In reply to the suggestions made:-

It's the only charger I have, apart from and old RS one which may not be suitable for NiMH and would take about 4 days to charge 2500mAh batteries. I might try it as it charges in pairs and has a separate indicator for each pair.

I have been running the batteries in a torch, trying to stop when it fades rather than when totally expired, to judge the state of charge and discharge them.

The charger does not have a discharge button but the camera has a discharge function that I can use when I eventually get to use the batteries in it

They should not yet need discharging but I have been playing partly to put them through several complete cycles to see if that improves things. It has not yet!

The charger does have a switched capability to charge just two so I will also try doing them in pairs.

I reckon I'll have to return it in the end!

 

[TrueBlue]

I've nothing complimentary to say about UniRoss products - despite their being widely available. Your experience confirms my opinion.

Years ago I switched to GP batteries for both primary and secondary cells (alkaline and rechargeable) and haven't looked back.

GP do a nice little unit which charges 1-4 individually monitored cells AA and AAA, and I think does NiCds as well. It's a fast charger, but doesn't cook the cells.
Comes with a mains power supply and a 12v car adaptor - so can use it on the boat.

May not be widely available but CPC will sell you one.

If you use loads of alkaline cells (and who doesn't) then the best way to buy them is in bulk packs of 40 £10 + Vat - 25p each. They keep for years - I'm just about to use some which are four years old....

Take the other stuff back and get a refund.

 

[ShipsWoofy]

My 'good' charger is an an-mann with single cell monitoring and discharge function.

On the boat I have a great unit which does all cells, I bought from maplin but see 7-dayshop have them for under a tenna now. Works very well for me, not many will do all, including PP9.

 

[nigelhudson]

Some of these chargers are dual-purpose - NiCad and NiMH. Has it got a selector switch?

 

[jwilson]

A lot of the chargers are set up for much lower capacity cells - I've had the same experience with a cheap charger. It will fully charge high-capacity NiMhs eventually but only after leaving them for many hours after it shows 'ready'.

I have a fancy 'Technoline' charger that works really well, and you can choose charge rate, discharge recharge etc.

As to buying bulk packs of alkalines as someone suggested - it's environmentally very unfriendly, and even the best alkaline won't match the power capacity of even a half-good NiMh. Alkalines are riht for low-consumption kit, things that run for a year or three on one battery. Anything used regularly shoudl run on rechargeables.

 

[VicS]

[ QUOTE ]
A lot of the chargers are set up for much lower capacity cells

[/ QUOTE ] I am trying to use the cells that came with the charger!

I have been playing along the lines that have been suggested. I tried charging the cells in pairs to determine if there was a dodgy one. First indiations were that there probably was but then that they were all ok. Yesterday I charged them in pairs apparently satisfactorily but today having discharged them again and tried to charge them all together the thing went into "maintence charge" mode when they were probably still only about 1/2 charged. However all 4 are now charged again (I hope) ready to play with again tomorrow.

After four complete cycles they do seem to behaving better but tomorrow is their last chance.

If I had bought the thing from a local store it would have gone back by now!

Anyone care to explain what negative delta V is. I assume it means that once the cells are fully charged the volts falls again a bit and that the charger senses this and shuts down.

 

[VicMallows]

Vic,

A bit late, but yes, the Delta -V method does work just as you describe. It was quite poular for NiCd in fast-charge applications quite a number of years ago.

I have not come across 'domestic' small chargers using this method. The majority use a rather crude timed charge (often 5hrs) before switching to a trickle. This can of course result in excessive overcharging if the cells are already fairly full. I even have a mobile phone that uses this method!

The most popular method of fast charging today seems to be temperature sensing, which many mobiles and cameras use.

The first thing I would do is connect a meter and observe the actual charge current current over a charging period. With the recent rapid rises in NiMH capacities, it would not surprise me one bit to find the cells were not matched to the charger they were supplied with!

Vic (M)

 

[VicS]

[ QUOTE ]
the Delta -V method does work just as you describe

[/ QUOTE ] Thanks. I did not realise it was on old system I thought it was a new idea. It seems to be increasing in popularity again anyway as the chaging times are reduced. Those with longer charging times seem to be purely timer controlled.

Anyway I eventually took it back and exchanged it (compete with batteries) It is early days but I dont think the replacement is going to be any better. I should have taken notice of True Blue's comments about Uniross equiment and bought a GP one from Curry's/ PC world. There is one very similar, but not quite so compact as it has the mains unit built in with just a detachable lead. Maplin have one "coming soon", a one hour fast charger thats a bit more expesive and one that takes a wide range of cell sizes but is consequently bigger.

All rather disappointing! Peed off about it in fact!