Slightly boaty - replacing sealed lead acid battery with NiMh

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We have a cordless vacuum cleaner - handy for the boat (there's the boaty connection!) whose sealed lead acid 6V battery has died - hardly holds any charge at all now. I wanted to replace it with some tagged NiMh cells. If I use 5 x 1.2 Volt tagged cells in series I can get 6V (Though was wondering about making it the half dozen to get 7.2V?).

The question is, once I've stuck them all together and fitted them to the machine, how to charge them? Is there an aftermarket charger that I can just put a cylindrical plug on the end of (like the SLA charger I have at the moment? Would the existing 6V SLA charger work? (assuming I don't go for 7.2V).
 
I have three Mascot automatic chargers. 3-6 cells @ 2.5 Amp, 5-10 cells @ 2.2 Amp, 10-20 cells @ 1.2 Amp. I believe that they are also available with a temperature probe. I switch a fan on when charging as the batteries do get quite hot. I bought them from CPC Ltd. I would certainly choose a temperature probe. if buying again. They look for a voltage drop that occurs when the batteries are fully charged. They also time out.

You may find a new lead acid battery cheaper!
 
I'd suggest looking for a new sealed lead acid battery first, they're quite cheap and it would be a simple replacement with no charging implications. I got one just a couple of weeks ago when my UPS stopped working, easy to source and buy online.
 
These guys are the goto people for batteries and chrgers in the modelling world. http://www.overlander.co.uk/batteries-chargers/nimh-battery-packs.html

They can make up a pack for you or sell you cells. Get tagged cells if you can.

A 2 cell lipo pack might be the cheapest option but you MUST get a suitable charger. There are lots of used ones around from RTF MODELS THAT PEOPLE HAVE CRASHED.
 
Ni Mh batteries should be fine for the job. Yes you might get more suction with the higher voltage battery at 7.2 volt. I found that "Sub C batteries gave the most claimed AH for cost from China. Not sure about the actual AH rating but they seem good anyway.
http://www.ebay.com/itm/8x-Sub-C-Su...US_Rechargeable_Batteries&hash=item2c6f052eeb
For charging I would suggest any plug pack producing more than 8v just fit a series resistor to limit the charge rate. This should be less than 1/10 of AH rate. So battery might rated at 4000maH so max charge rate should be less than 400ma. Less is better but makes recharge time longer. And of course depends on rating of the transformer/plug pack.
The resistor value is given by the formular voltage (difference between battery voltage and supply voltage) divided by the current in amps. So a typical resistor would be 3v drop froma 9v transformer at 400ma needs about 8 ohms with a power dissipation rating of more than 2 watts. It will get hot. So not a tiny resistor but a bigger wire wound type.
Get a few resistors and check actual charge current with a multimeter on amps range.
As said Lithium ion batteries are even better but must be carefully charged at a low rate. They can burst into flames if overcharged. I think a rate of about 1/30 of AH raing hould be safe however provided you don't leave it on acharge and forget it. In any case the SLA charger you have is not suitable for other types of batteries if just connected up. You need to check with amp meter what is going on. (and limit current) good luck olewill
 
Best to replace like-for-like. Lead acid replacements are cheap from Ebay, give plenty of current and will match your existing charger. To replace with NiMH will cost more, have the problem of making a pack to fit the space, the cells will need to be soldered together, you will need a different charger and not all NiMH cells give a high enough current. Sometimes the simple, easy solution is the best!
 
Thanks all, I guess I'll not proceed with the plan and will probably just get another SLA. The current one is an easy shape to source - 6V 4Ah. I did stick an ammeter in series with it and switched it on. There was a spike of about 13A for half a second or so while the motor got up to speed and then a steady-state consumption of 6-7A, a bit more than I'd imagined really.

The problem is that the rest of the family seem to be firmly convinced that all cordless appliances come pre-filled with "electricity" for life so it spends a lot of time sitting there, deeply discharged, waiting for the "electricity fairies" to come and put some more electricity in it and I get fed up buying new batteries. Probably easier / cheaper to educate the family, I guess.
 
I've found it a good idea to have a 'battery charging day' on the first of every month - it's surprising how many things need charging these days, from laptops/netbooks/tablets, to torches and bicycle lights, to TV remote controls. Leaving stuff discharged is a sure way to kill the batteries, and doing everything on the same day ensures nothing gets left too long. I also label the chargers with Tippex to make sure I don't get them mixed up!
 
greenalien said:
I've found it a good idea to have a 'battery charging day' on the first of every month ... I also label the chargers with Tippex to make sure I don't get them mixed up!
Click to expand...

I was in full agreement with you until I saw the bit about Tippex. UGH!

Do yourself a favour and get one of these:
http://www.brother.co.uk/g3.cfm/s_page/215760/s_level/225380/s_product/PT1230PCZU1

Incredibly cheap from this seller on Amazon:
http://www.amazon.co.uk/exec/obidos/ASIN/B001IKKIVM/dolcetto-21

Or for a standalone unit, one of these:
http://www.amazon.co.uk/exec/obidos/ASIN/B002USNXDQ/dolcetto-21

Incredibly useful at home and on the boat.
 
I already have various labelling machines, both Dymo and Brother - but they aren't very good for marking rough surfaces, hence the Tippex - writes in white, ideal on black chargers - and cheap to buy!
 
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