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rechargeable batteries

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default rechargeable batteries

Post by Dandelion on 26th February 2011, 10:30 pm

I've been using rechargeable betteries for years now, but to be honest for some things they just don't seem to hold enough charge. They'll run a clock, but for anything like a portable Cd player they aren't any good at all. I've tried several chargers - our present one has a light which tells you when the batteries are fully charged, but when it goes on they are not fully charged. I have been told about the 'memory' that batteries have, and I only charge them when they're completely flat. Does anyone else have the some experience, or can you recommend a charger which does the job properly? We mainly use AA batteries.

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default Re: rechargeable batteries

Post by Compostwoman on 26th February 2011, 10:53 pm

Part of the problem is that many appliances say the battery is low and won't use them any more but the battery is not actually fully discharged. So when you recharge it from that state, it shortens the life and reduces the effectiveness...(sic)

Things I have found work

I would recommend a charger which totally discharges the battery first ,before recharging it. We have several of these and they have helped to prolong the life of batteries enormously. I will look up what make they are and report back.

Also many cameras ( other appliances also?) have the facility to totally discharge batteries, even when it is not prepared to continue operating with the battery any more,.,..when I found that out it made my battery consumption in my camera drop enormously!

Finally, buy expensive rechargable batteries. The cheap ones really DO NOT last many charging cylcles. More expensive ones seem to last a longer time.

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default Re: rechargeable batteries

Post by mark barker on 26th February 2011, 11:04 pm

I get through plenty of batteries in my bike lights, and I've used a fair few cheap chargers and had problems with them killing batteries. I switched to this one and its made a huge difference.

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default Re: rechargeable batteries

Post by Sparhawk on 27th February 2011, 2:15 am

We have a battery charger that charges normal batteries, I got it for a couple of quid in a charity shop, & although it doesn't give you brand new batteries each time, it does give normal batteries a couple of extra lives...

One of the Young Squire Sparhawks jobs is supposed to be to try to charge them each month...

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default Re: rechargeable batteries

Post by Mike on 28th February 2011, 10:51 pm

Type? It makes a big difference. And of course you need the appropriate chargers cfor the different types (or one that can recognize/handle the different types) and as noted some will even "condition" batteries and/or leave them on trickle charge. But first type ..........

NiCd ---- these are what you are perhaps most familiar with. Can be recharged many times (500-1000) but have severe memory issues. They will not hold a charge, losing perhaps 1-2%/day.

NiMH ---- newer (actually the technology is older since the "Edison battery" was of this sort), do not have as severe memory problem so can be topped up. Can be charged 500-1000 times. The standard ones lose charge 2-3%/day but there are now "charge holding" ones that lose only about 50%/year (they are pricey, but now can be used in torches and headlamps and other gear you need to be ready for emergencies.

Rechargable alkaline -- Maybe you don't have these? Only good for ~25 full charges and must never be run all the way down and left that way, especially not slowly (opposite of NiCd). No memory problem and hold charge just about as well as ordinary alkiline batteries (years). The sort you want for that torch that lives in the glove compartment of your car, etc. Do not put one of these in a clock! (it won't recharge if slowly run down like that). I am assuming that unhappy consumers unwilling to learn to use different type batteries for different applications)led to them being dropped by Rayovac here but I can still get these made by an outfit in Canada.

Since we used to use just rechargable alkaline* I have enough left for strictly emergency applications and have been phasing them out replacing with the "charge holding" NiMH for devices we use more regulary (our headlamps, the flashlights we use during power outages, etc. --- as opposed to the one in the glove compartment, etc. that might not get used for years but must work when called upon).

* Leaving aside clocks and alarms (these need to be ordinary alkaline for slow discharge over years) >90% of our batteries used in emergency lights (headlamps, torches, lanterns, etc.) and so never were "not ready when you need then" NiCd.

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default Re: rechargeable batteries

Post by Chilli-head on 1st March 2011, 10:23 am

NiCd batteries are tricky things. The true "memory effect" occurs only under very special circumstances and is highly unlikely to be seen in normal domestic use. What you do see that looks very like memory effect is a result of individual cells that make up a battery becoming out of sync in their charge level. With N-Cd, this is quite likely with little used batteries because they have significant, and cell-to-cell variable, self discharge rates.

When the cells are charged as part of a whole battery, this causes some cells to reach full charge before others, Depending on how/whether the charger decides to stop charging, either some cells are left incompletely charged, or others overcharged. If a battery with some cells fully charged and others not so is then discharged, at some point the least well charged cells will become empty, and the battery voltage drops rapidly. If you then continue discharging the remaining good cells, the stronger cells will effectively reverse-charge the empty ones, destroying them.

So:

1: use a good charger which stops without overcharging
2: Charge slowly, and fully, to get the pack completely charged.
3: They are best charged as separate cells, not as a complete battery - some chargers do this.
4: When "conditioning" a NiCd by completely discharging/recharging, it MUST be done as individual cells, not a set of cells in series as a battery, or the weaker cells will be damaged by reverse charging.

In my experience, #1 above is key. Bad charger designs destroy batteries. Unfortunately they are the norm. My old cordless screwdriver had a "charger" which was just a transformer and rectifier diode, and a thermostat. It relied on the resistance of the transformer windings to provide a "regulated" charging current, and simply stopped when the batteries were cooking ! They did not last long.
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default Re: rechargeable batteries

Post by mr_sfstk8d on 1st March 2011, 2:13 pm

Have to agree with what Chili and Mike says about wonky chargers.

Chili, I see what you mean about individual cells. This makes sense in devices that have multiple connectors, such as the 3 and 4 point flat pack batteries for some cameras, camcorders, etc. While it's, technically, true for 'conventional' round batteries, it's rather impractical though, lol. I don't know any home device that can disassemble AA batteries, charge the cells idividually then reassemble them. But in practice disassembly is not necessary, since they're an integral series circuit voltage supply essentially.

OK, but more about individual cells and quality power supplies for the home consumer. Some (many) chargers require batteries to be charged in pairs. That's because the power supply charges them both together in ONE circuit as ONE battery. In this case, what Chili says about one cell being undercharged and one being overcharged is highly likely. This is especially problematic if the batteries were not equally discharged before hand. And even more so if they are not of the same rated type (1800 mAh, 2500 mAh), even though they are the same SIZE.

Ways to get around this:

Fully discharge batteries before charging. To do this, I use a flashlight (torch) that is NON-LED. Incandescent bulbs will discharge whatever remaining voltage in the battery. LED require a minimum voltage to run. Once the potential drops below this, no more light. There's a complex silicon doping, xener effect explaination I won't get into here. Suffice to day, if you want it dead, use an 'old school' bulb. This may be considered a bit of excess only needed for deep reconditioning, and admittedly, I don't do it every time, but it is usefull.

Charge the same type of battery in pairs at the same time. Pair up batteries of the same mAh (milliAmp-hours) rating. Preferrably, same manufacturer, as there may be minute differences one to another.

Best choice, use a charger that does the battery conditioning for you, monitors charge levels, and charges batteries individually, not as a bank. I use an Energizer model for this, the model no. escapes me. It has a 3 colour light at each bay for status. It also has a USB port in the bottom for charging USB devices (iPod, etc.) It can charge one battery at a time, up to six, can do a conditioning cycle if needed, and can tell me the overall health of a battery, whether it's good to take and hold a charge or not.

Your Milage May Vary, Void Where Prohibited, Consult a Physician, etc.
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default Re: rechargeable batteries

Post by Mike on 8th March 2011, 8:21 pm

Never had a charger that didn't manage each cell individually so I don't know what sort of cheap chargers you folks are talking about.

Our current charger can take any of the common size batteries and any of the common chemistries and will detect if a NiCd needs "conditioning"*. Take a look at the models offered by some outfit that handles this sort of gear (say "Sundance Solar" -- you perhaps have the equivalent in the UK?). Really good chargers aren't that expensive considering the cost of the batteries and how much longer they will last if you have a proper charger.


* And it would run off a solar panel instead of being plugged into an AC outlet or the 12v DC of a vehicle but enough solar panel to operate the charger would have cost an additonal several times what the charger did.

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default Re: rechargeable batteries

Post by Chilli-head on 9th March 2011, 10:02 am

Mike, I guess you are right that many chargers that take, say, 1 to 4 AA cells probably manage them individually. I also guess that the thing to watch out for is that you get one that can charge any number of cells, not just all 4 (or whatever) at once - a sign that it is likely to be charging the whole set in series.

Lots of things that come with rechargeable battery packs have a charger that recharges them as a pack - the connections to individual cells are not accessible externally. A good charger for this type of pack - like a DeWalt cordless drill one I use - checks the state of the battery first, rapid charges until dv/dt or temperature rise indicates some cells are approaching full charge, then drops back to a safe overcharging current, dropping to a maintenance trickle when the terminal voltage reaches about 1.45v/cell. That way all the cells end up fully charged without serious overcharge to the stronger ones.
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