Battery Question

[grouchyturtle]

I have a small LCD for my video housing that runs on 12v.
Between needing a very small battery pack, because of available room in the housing, and the price for 12v NiMH packs, I decided to go this route instead.

Bought a battery holder from Radio Shack and bought (8) 1200 mah AA NiMH batteries. The pack connects with a standard 9volt style clip.

I have 2 chargers.
One is a Alltek combo charger for AAA - D, and 9v
The other is a MAHA C777 universal charger

I was thinking instead of using the combo charger and only charging 4 batteries at one time, why not just keep the batteries in the holder and connect the holder to the universal charger. I was thinking of maybe even getting some large shrink tube, and preminently wrapping the batteries in the holder.

Any reason not to do this?

 

[Charlie99]

II was thinking instead of using the combo charger and only charging 4 batteries at one time, why not just keep the batteries in the holder and connect the holder to the universal charger. I was thinking of maybe even getting some large shrink tube, and preminently wrapping the batteries in the holder.

Any reason not to do this?

It should work OK, but it is less than optimum for a couple of reasons.

Unless your cells are perfectly matched (and they never are), charging in series (like the MAHA) is less than optimum. The best chargers will charge and monitor each individual battery.

Wrapping the batteries together in a shrink tube will also increase their temp while charging. Higher temps will shorten the battery life. Nothing dramatic, but it will shorten it a bit.

It's a tradeoff between convenience and a little bit better performance.

 

[miketsp]

Just a comment. You are aware I take it that NiMH cells are NOT 1.5V per cell, they are nominally 1.2V.
So you need 10 cells for 12V.

Example:
http://www.batterieswholesale.com/battery_packs_10_cell.htm

 

[miketsp]

I have a small LCD for my video housing that runs on 12v.
Between needing a very small battery pack, because of available room in the housing, and the price for 12v NiMH packs, I decided to go this route instead.
..snip..

When you buy a battery pack you have 2 advantages.
a) The cells have been selected from a matched lot so will charge and discharge more consistently in series.
b) The inter-cell connections are permanent so you will have far less bad contact problems over the life of the pack, especially in a marine environment. I'd buy the pack, there are plenty of types to choose from.

 

[grouchyturtle]

Wrapping the batteries together in a shrink tube will also increase their temp while charging. Higher temps will shorten the battery life. Nothing dramatic, but it will shorten it a bit.

The MAHA does have a little magnet that goes on the batery pack to sense the temperature though, and I believe adjusts the charge depending on the temp.

Just a comment. You are aware I take it that NiMH cells are NOT 1.5V per cell, they are nominally 1.2V.
So you need 10 cells for 12V.

CRAP!

Well I was able to get it run with a 9volt. Got about 10 min., and it was an older and used battery too. Also the LCD has video and audio inputs. I'm going to make a new custom cable that will only connect to the video, so that should draw less power.

A 9volt NiMH is only 170mah The AAs are 2100mah and 9.6 volts. So shouldn't they still get me about 2hr of run time.

I'd buy the pack, there are plenty of types to choose from.

Problem is, as I said, it's a very small housing. I couldn't find anything small enough, so I'd probably have to pay through the you know what for a custom pack

 

[DA Aquamaster]

Check with an electronic supply house and buy rechargeables with solder tabs. Then you can actually reduce the size further by eliminating the battery holder and you solve the potential bad connection issues as an added bonus.

Shrink wrapping is also fine as long as you use something fairly thin like mylar rather than just large heat shrink tubing. A small fan blowing over the battery pack will also more than offet the insulating value of the shrink wrap if you are worried about shortened battery life. Personally, my thoughts are that I can almost always make 3-4 packs for the same price of buying one, so if they each only last 80% or 90% as many cycles, I am still WAY ahead.

Don't sweat the 1.2 vs 1.5 volt difference too much. In general a rechargeable will have a slightly lower voltage but will maintain a more even voltage over it's discharge cyle maintaining nearly the same 1.2 volts to the bitter end where it suddenly falls off. An alkaline in comparison will start an immediate but gradual decline in voltage until it slowly reaches the point of inneffectiveness.

If the device runs ok on the lower voltage associated with the rechargeable cells, you are good to go. If not, just add another cell or two to the pack to make up the difference.

 

[grouchyturtle]

Would ther be any reason not to just solder the regular AA batteries I already have together...other than it would obviously be a bigger PITA than the ones with the solder tabs?

Do you just use short pieces of wire to solder the batteries together?

 

[jonnythan]

You can mess up NiMH cells pretty bad if you're taking a soldering iron to them. Just put an 8AA pack in series with a 2AA pack. Charge them as a pack for simplicity. You can replace all 10 batteries for very little money, so it's not worth the time to charge them separately.

 

[miketsp]

Most battery packs are spot welded together using nickel strip for contacts. However, not everyone has a capacitive discharge spot welder. Soldering directly to the cells is dangerous for the cells. It is easy to melt or disturb the safety vent, thwack the seals, or cause internal shorting if the heat is too high. This damage might not be noticeable until later.

Extract from:
http://www.powerstream.com/BPD.htm

 

[jonnythan]

Now that I'm thinking about it, 8 cells might be preferable to avoid killing a cell. Each will cut out at a higher voltage if you run the thing until it goes off vs if you had 10 cells. for example, if the thing stops working at 8.6 volts, that's a cut-off of about 1.1V per cell vs 0.86V per cell for 10 of them.