Multimeter batteries

[steve30]

I have a Thurlby 1503 which I have been using for a good year or so. It has 6x C cells which came with it when I bought it, and they are finally on their way out.

I'm pondering having a rechargeable battery of some kind, but I don't have a charger for C cells. Also, a lot of rechargeable C cells on the market seem to be the same capacity of AA cells, so I can't really see any point in using cells like that, as I may as well just use two sets of AA cells in parallel.

I was also pondering the possibility of using a Lithium pack of some kind. Anyone got any suggestions on what to look for?

The meter is rated to take 21mA, but I managed to measure it as using 45mA. I do sometimes leave it on for long periods of time, so I want any kind of battery to last long so the meter is ready for use whenever.

Any suggestions are appreciated.

[SeanB]

Eneloop cells and the C cell adaptors will probably work out the best, as they are reasonably cheap, and have a long shelf life before you have to charge them again. A flat discharge voltage as well will help with the life, as they will be a constant charge until near empty. 2 4 cell chargers and a set of 8 cells means you have 2 to use in another device, and can charge all 6 at the same time.

[steve30]

Eneloop cells look reasonably good. I might try them when my existing AA rechargables die.

Not too keen on the idea of using 6xAAs in the multimeter unless I had two sets of parallel ones. Only reason being that there is quite a lot of space inside the multimeter, so I thought I might as well get as many mAh in there as possible.

Also, the weight of the C cells is actually quite beneficial

[SeanB]

If mass is needed add a ballast weight to it to get it right, or some soft silicone rubber feet to hold it to the surface.

[retiredcaps]

Eneloop cells and the C cell adaptors will probably work out the best

I have used eneloop AAs in D cell (generic sort by lowest price ebay) adaptors and they work great.  Obviously, I don't get the same run time, but I have two sets.

[mariush]

Have you considered supercapacitors?

You can get something like 100F in the size of a C battery : http://www.digikey.com/product-detail/en/BCAP0100%20P270%20T01/1182-1012-ND/3079276

Get 6 of those, wire 3 in parallel to get 2  x 300 F 2.7v , then wire these in series to get 150F 5.5v.

Add a boost regulator on a tiny pcb to get ~1.5-5.5v up to about 6.5-7v  (the service manual says minimum is 6.3v, 6 x 1.2-1.5v = 7.2-9v) with maybe a pi filter to smooth out the output ripple and you have yourself a few hours of battery life.
As for charging, you can simply use a usb charger or the usb on your computer (but with a bit of current limiting because those supercaps would otherwise draw in several amps of power - I'm playing right now with a 50F 2.3v capacitor and draws about 2.2 A when empty) and the "batteries" should be charged within 5-10 minutes to a reasonable level, maybe about 70% of total capacitance... in about 20 minutes they would be fully charged.
 

[steve30]

Supercaps are a nice idea.

I do keep meaning to try and get some supercaps to play with