Easy Homemade 3-Battery Charger? (AA/AAA NiMH)

[PerryScanlon]

A friend of mine was complaining about his headlamp using 3 batteries even though batteries come in packs of 4, and I've been wondering how to charge sets of 3 batteries for my headlamp. It looks like there's a lot of interest in this subject on the whole Internet because flashlights and headlamps commonly use 3 batteries to reach the voltage requirements of white LED's. Here's a sketch I made of a 3-battery charger design:



The theory is that when a battery is at 1V and basically dead, it would charge at a medium rate like 500mA, but as the voltage approaches 1.35V the current goes to zero. Thoughts? It looks like there's a lot of subtleties involved in charging batteries. Is this design too simple? I realized after sketching that the power supply needs to have a higher current rating.

[PerryScanlon]

Here's a ghetto charger that's even easier and cheaper to build:



I think if the resistors and diodes are chosen carefully, and voltage measurements are done everywhere with sets of dead and fully-charged batteries + an open circuit voltage test, it could be safe and reliable. I'm not sure about mixing 2 really dead batteries with one almost-full battery, but I think it would work with normal flashlight and headlight use if the battery batches are not mixed.

[march.brown]

I would simply buy a reasonable four bay AA/AAA charger off Ebay or Amazon ... It saves a lot of messing about and would look better ... There again , some guys like to build their own chargers ... I'm waiting for some bits to arrive from the Far East to complete my 18650 Li-Ion chargers.

You might be able to get the charger PCBs ready made (on the internet) with the correct charging algorithm ... It's only a matter of plugging into a mains (USB) supply and feeding the charger outputs to your batteries ... I will be housing a Li-Ion battery in one side of a double battery holder ... The PCB will be epoxied into the other side of the holder ... I will be making up two of these single chargers and will be feeding them from a 1A USB wall supply initially , till my 5V 4A PSU arrives.

Anyway , Good Luck.
.

[czAtlantis]

No you can't use this kind of charger from first post. Ni-mh are charged "by current" not "by voltage" like liions.
If you really need simple charger, use LM317 as constant current source. Even in datasheet there is simple charger circuit using LM317 and one resistor. That is all.

Btw I use this for low current charging and terminate manually after some time. Sometimhes Maha is not so freat, specially in lower capacity/older cells.

[PerryScanlon]

Thanks March. I did a search, and the independent bay chargers are more affordable than I thought they would be.

CZ, you got me thinking about that circuit more. It sounds like strict voltage regulation might be too inflexible to account for the nature of different NiMh batteries. If I'm understanding correctly, 1.35V regulated might get a battery stuck half-charged while trickling at 1mA, whereas 1.4V might overcharge and damage another battery after it's voltage starts decreasing?

I understand that charging to 100% in a short amount of time is complicated and best done with a digital charger with a temperature sensor, and a trickle charge is another method that is reasonably safe for a battery, but I'm wondering if a basic analog charger that charges to 90% by tapering the current would prolong the useful lifetime of the batteries?

Probably I'll build one or both circuits. If anybody is interested in hearing results and measurements I can post later...

[march.brown]

Thanks March. I did a search, and the independent bay chargers are more affordable than I thought they would be.

Hi Perry ... I use several chargers , but my favourite one is the (recently bought) Powerex/Maha MH-C9000 ... If I need to charge more than four batteries , I have a Hama Delta (fan-cooled) charger plus my original favourite (un-named) charger ... All are four-slot models with individual slot control ... The Hama has LEDs whereas the other two have LCD displays ... Till I had the C9000 , I tended to use the unknown make charger which also has a discharge facility ... The C9000 is large and fairly expensive , though Amazon UK had the best price.

Powerex/Maha make a cheaper charger than the C9000 and my un-named charger is identical to it ... There are similar (modestly priced) LCD chargers on Ebay that don't have the discharge facility and I find it comforting to see the bars on the display moving up as the charge progresses ... The C9000 LCD display gives you time taken , mA charge rate , mAh figure and also cell voltage for all four slots ... It also says "DONE" when the charge ends ... You may not actually want all this information though.

For the price , I would go for one of the four slot LCD chargers off Ebay ... You could get four (or more) of them for the price of the C9000.

It all depends how much you want to spend.

Good Luck with your purchase or with your charger building project ... Or Both !
.