36 volt battery charger

[silversurfer]

Hi Folks, just a quick query, I am about to build a back up battery for my Lithium battery'ed Torq, from 'D' cells sold by Lidel supermarket (on offer this week). They are Ni Mh, 4500mah type. I am looking at a 36v Nicad (cordless drill?) charger on ebay. Would this be suitable for Ni Mh cells ?
I did recently buy the Lidel 'dual' Nicad/NiMh charger, but it only does 4 batteries at a time, which means stripping the pack each time!!!
I also plan to put an extra cell into the 36v pack to keep the nominal voltage up, so it could be 38ish volts, not sure if an 'electronic' sensed charger would be upset by this ? Silversurfer

 

[flecc]

You are opening a big can of worms here:

1) This sort of cell rarely has an adequate discharge capability

2) NiMh cells in series need to very carefully matched and preferably from the identical production run.

3) NiMh packs need a thermistor of the correct specification and placement to monitor the tempertaure gradient throughout charging.

4) NiMh packs need a charger with a thermistor monitoring circuit that matches the characteristics of the fitted battery thermistor, particularly the resistance and beta factor.

The result of getting things wrong can be the burnout of the pack and a potential for a fire with incredibly high temperatures reached. E-bike NiMh batteries are expensive for a reason, the cells included often costing £10 or more each on the retail market, and it's the problems in producing the packs reliably which has prompted manufacturers to rush into lithium instead.
.

 

[silversurfer]

Thanks flecc, thought I might have a problem or two! I bought them at the back end of last year (but I'v been too busy to finish project!) after reading your interesting post on a backup battery using AA cells, I fig. that 'D's should have a bit more delivery capacity.
With a bit of a heart problem I can't afford to slog for to long and a backup for the last couple of miles on my local 'Wirral tour' was tempting.
Looks like I will need to charge them on the 'official' Lidel NiHi charger, 4 at a time. I assume this has all the ness. thermister bits built in, it certainly flashes red LEDs (to auto diagnosis/ refresh) it has delta U cutoff, single slot ? monitoring, green LED's to inform end of charge.
I assume the danger is in the charging and if so the 'Lidel' charger should be OK ?
Could I connect the 38v. pack in parallel with the Lithium battery(about 20 months old used approx. twice a week) to supliment the range so as not to put too much stress on the 'D' pack ? Thanks silversurfer

 

[flecc]

Charging is indeed the greater danger point, so it's best to use a correct charger for the pack that has a peak voltage cutoff point as well. With that you could avoid the complication of using a thermistor, substituting a 10k resistor to allow the charger to operate. About the cheapest way to get a correct charger is by import from Powerstream in the USA due to favourable exchange rates and using the cheapest postal service, about £45 lowest.

Using the 4 cell method might be ok short term, but with the imbalances that could occur, the cell life could be short, but as long as the backup battery is used in isolation and you're prepared for it giving lower power, it could be useful.

I don't think you should try to run a home built pack in parallel with the lithium battery without protection circuitry though. You could do something nasty to the expensive lithium battery, and running any batteries in parallel without protection against cross currents is generally regarded as inadvisable.
.

 

[Tiberius]

Hi Silversurfer,

As flecc says don't parallel the batteries without a protection circuit. The good news, however, is that the protection circuit is dead easy. With a pair of Schottky diodes you can connect batteries up this way, and it really does help the bike.

Have a search through some of the threads on this forum for Schottky diodes or battery mods and you will see what we mean.

Nick

 

[silversurfer]

Thanks guys, invaluable info. As a retired motor engineer I can cope with basic electrics but electronics is a step slightly too far! I will investigate your tips and I think the Lidel dedicated charger will sufice for now, it has a max output of 1 amp on D cells taking 2 to 12 hours to charge (dep. on A/H of cells) My Litium battery is loosing range now, probably down to 16 miles mixed pedal/motor assist.
I have a friend in the local golf shop and he can supply me with plenty of slightly 'tired' but still sound hill billy 12v 12a/h batteries. I am toying with the idea of three or four in my pannier bags, or building a little trailor from a nice upmarket kids buggy (with inflatable tyres!) I found when I was dumping some rubbish at the tip, true recycling!!!
Thanks again for your promp reply, silversurfer.

 

[Jeremy]

If you need any further confirmation of the possible hazards of building a NiMH pack without including effective internal pack temperature measurement for charge control, then take a look at what happened to me a few months ago:

http://www.pedelecs.co.uk/forum/electric-bicycles/1444-battery-fire.html?highlight=battery+fire

Jeremy

 

[silversurfer]

Having read jeremy's story I must rethink my project!
Ithought Ni Mh batteries were the reliable work horse of the battery assisted bike. I bought a fairly cheap nimh AA 'kit' (8 batteries and a fast charger) a couple of years ago for general use, torches etc. I found it got the batteries very very hot so I gave up using it. I see now what may have happened if I had left it overnight. These domestic ones are sold without any strong warning about possible dangers.
I think my old sealed lead acid golf batteries are becoming more appealing by the hour ! silversurfer.

 

[Jeremy]

I wouldn't be too worried, provided you have some form of reliable charge reduction/cut-off that's triggered by cell temperature. My error was in thinking that I didn't need to sort out the thermistor wiring for an initial, low rate, charge. As I discovered, I was very, very wrong!

The annoying thing is that I had already fitted two thermistors inside the pack, so had I taken the time to finish the wiring properly, instead of rushing ahead with the initial charge, everything would have been alright.

It's fairly easy to insert thermistors into home made packs and get them to work with existing NiMH chargers. There are a couple of threads somewhere on here detailing the thermistor type needed and the way to connect it, I believe.

Jeremy

 

[silversurfer]

Thanks jeremy, I will rethink my design to build in some safety device. I have only the domestic 4 cell Lidel nicad/nimh charger and at this stage I did not want to invest in an expensive charger, especialy if the 'cheap' 4.5ah Lidel cells turn out to be not up to the job. I can charge them up slowly (4 at a time) and try them out.
My design is to 'load' them into a plastic water tube, (sold by builders merchants) like an 18" torch ! Three of these give me my 38 volts.
Anyway I am away for three weeks from tomorrow so everything is on hold, but at least, following the forum's advice I will not be wasting money on an unsuitable ebay nicad charger.Bye for now, SS