Ni-MH battery chargers


Everyone knows how to charge AA batteries: you just stick them in the charger and that takes care of it. That’s what I used to think, until I realised I was only half charging my batteries, even when my fast charger said they were full. This page shows how I discovered this. It then outlines a method for fully charging a set of Ni-MH rechargeable batteries using a device that many people would consider outdated junk (and that you can buy on ebay for next-to-nothing). But first here’s an outline of the problem I was having with my Ni-MH batteries and the bike lights they powered.

“This lights draws so much power it’s unusable”
It all started one Christmas time when a generous employer gave me a £100 voucher to spend at Wiggle. Awesome Christmas present, I thought, I’ll buy a really bright light. The light I chose was a Hope Vision One, and it was really bright. But I noticed it would cut out part way through a two-hour ride. It drew so much power, it was almost unusable; what a terrible design flaw, I thought. Thing was, I loved that light because it was so bright, I could ride confidently at night at daytime speeds. And so I persisted. The capacity of my batteries must have been depleted from age, I thought. I’d had those Maplin AAs five years. So I bought a new set, which gave better results, though I took my spares along and changed them at the roadside when my light went out. This wasn’t a perfect routine, but it was the best that could be done, I thought.

While I was blaming the light for having a design flaw, or blaming the batteries for just being old, I’d been charging the batteries the same way, in a simple 10-hour charger I’d bought new for no more than £15. Sometimes those batteries would come out of the charger warm, sometimes cold, and occasionally they’d be hot. Hot was bad, I knew, so I’d check them form time to time when they in the charger to catch them before they got hot. If only there was some way of automating this.

When is a smart charger not smart?
I’d heard about intelligent chargers and liked the idea of a charger that switch off when the batteries were full, but was surprised by their prices. £50 for a device that charges £12 worth of batteries, seemed excessive. It wouldn’t be worth it, I decided. Instead I chose a smart charger that promised to charge my batteries in just two hours, in a ‘special deal’ at one of the discount supermarkets. What made my charger ‘fast’ was an output current of 1000mA. Not just smart but time-saving too. A shrewd investment, I’d thought.

I realised that charger was not particularly smart after some of my batteries came out too hot to touch. When this happened a few more times, my investment seemed less shrewd (and I doubt I was getting better run time in my super-dooper bike light). But still I had a fast charger, which could be useful, so I kept it. And after a while, I realised that I did not need to check on my batteries if I set a plug-in timer to cut the power after an hour.

At this stage my understanding of battery charging could be summed up like this:

  • Fast charging generates heat
  • Heat from overcharging damages batteries internally and reduces their capacity
  • There were better chargers out there

I still didn’t know the true capacity of the batteries I had been using. I didn’t know whether I was fully charging my batteries, I just assumed I was. I assumed, ‘they must be hot because they’re full’. I would find out just how undercharged my batteries were after I bought another piece of kit.

A charger with digital meter – a Uniross-badged Technoline BL-700N
I eventually decided to invest a little more, and chose a Uniross-branded charger. This seems to be a clone of the better-known Technoline BL-700N. This device revealed how poorly my other battery chargers had been performing.

It meters battery voltage while it’s charging, and while I’d previously been happy with a final voltage of 1.2V, and considered 1.3V to be good, this charger took the same cells up to 1.45V or, in some cases, 1.48V. It charges by applying a current of 200mA until it senses the cell is full at which point it steps down to a 10mA maintenance charge.

It also has a capacity test mode. This takes a charged cell and discharges it at a set rate until the voltage drops below 0.9V. The charger calculates the capacity (as the discharge current multiplied by the number of hours that current was applied; for example, 100mA discharge for 20 hours indicates a capacity of 2000mAh). It stores and displays this result.

First time I used it, I saw that my previous chargers had been charging batteries to 50% nominal capacity. The cells rated at 2400mAh were delivering only 1200mAh. The penny dropped: this is why the Hope Vision One was cutting after an hour and a half. This was also why my Ni-MH cells hadn’t been working in an AA-powered camera. Measuring battery capacity allows me to know which cells can be used in a high-drain device like the Hope Vision One, and I’ve found it useful to label my cells with their ‘measured capacity’.

An inexpensive trickle charger
I subsequently found I was able to get up to 10% more capacity by taking the cells out of the intelligent charger when they displayed a voltage of 1.4V and topping them off in a separate trickle charger. My trickle charger delivers 45mA which is sufficiently small that the cells remain cold throughout. I can leave them on charge in this device for three days and they won’t heat even when their voltage reaches 1.49V. Since trickle charging needs no supervision, it’s as convenient as intelligent charging, albeit slower.

Trickle chargers are available inexpensively as used items on ebay, where they’re listed as “vintage chargers” or “old chargers” for AA Ni-Cad batteries when people have found them in the loft or the garage. The charge rate is displayed on the charger label, anything between 35mA and 50mA is useful. These devices were standard chargers for early rechargeable batteries (which had a capacity of less than 800mAh); they are 50-hour chargers for modern Ni-MH cells. Fine for charging a spare set. In fact, a trickle charger and a spare of 2300mAh Ni-MH sells will cost less than an intelligent charger and so could be good substitute – although it takes longer, it couldn’t be easier to use as it requires no user involvement. The trickle charger is also good for topping off batteries that have just had a short period in a timer-controlled fast charger. It could also be left plugged all the time, perhaps on a clock timer, to keep a fully charged set of cells charged and ready to use.

So here’s where I’m at with my understanding of Ni-MH battery charging:

  • Fast charging is acceptable as long as you stop before the battery gets hot
  • Automatic power-off can be implemented on any charger using a plug-in countdown timer
  • Stopping a fast charger early means the battery will be only partly charged
  • Trickle charging can take Ni-MH battery voltage to 1.49V and beyond nominal capacity
  • Trickle charging at 45mA or less leaves batteries cool (indefinitely)

I pretty much run my life on Ni-MH batteries, using them in bike lights, a handheld GPS receiver, torches and a couple of cameras. There’s a set in use everyday and a set on charge most of the time. While it took a good deal of trial and error to arrive at the charging set up I now use, it was worth it to find a system that’s inexpensive, easy to use and that needs no supervision.

Of all the pieces of kit discussed here, the vintage trickle charger is the single best investment I’ve made. My second most used item is the plug-in countdown timer which cuts the power to my fast charger (after 60 minutes) to prevent overheating. I use the intelligent charger for battery analysis every six months and when I acquire a battery whose capacity I need to measure.

For cable management, I place the chargers in a box and connect them to a four-way extension socket. The chargers and their flex stay in the box and there’s just one 13A plug in the wall socket. I chose a wooden box with the same footprint as the other storage boxes I have, so they all stack when they’re not in use.