Rechargeable batteries- Lead Acid vs. NiMH (pignose)

Need some info about replacing some rechargeable batteries.

I recently bought a Pignose Hog 20 portable amplifier, which I put a strap on so that I could walk around while playing. (It was for a strolling-among-the tables dinner gig.) The amp basically worked fine, but it contains 2 really heavy lead/acid batteries which made it awkward to carry around. I was wondering if it would be possible to replace the lead/acid batteries with NiMH rechargeables. Here are some details:

Current Batteries- two 6V 4.0 Ah /20HR sealed lead acid batteries wired in series

Amp-(Info from manual)-Capable of producing UP TO 20W power…
Operates for several hours on a full charge
(not very specific, I know)

Possible replacements- eight to ten C-size NiMH batteries
(rated 1.2V, 3700mAh) wired in series

I never run the amp above half volume, so I thought I could run in on 8 batteries rather than 10, and I don’t need to use it for more than an hour at a time, so the slightly lower Ah rating should be okay.

Are there differences in resistance and so on which prevent this replacement?
Is it okay to run so many NiMH batteries in series?

Without going in depth into your question I’ll pint out a couple of things.

  • You can connect as many cells in series as you need and there is no limit or problem with doing that

  • I recommend you use 10 to get your nominal 12V (or even 11) but not any less.

  • In general, D size batteries will give you more bang for the buck as they are more common and cost about the same as C size but hold much more charge in relation to price.

I carry a pack of 4 Ds on a belt for my digital camera. I think I could carry three packs just as well but then you have to add your amplifier which I have no idea how much it weighs…

My practice is to always carry the biggest batteries I can. Most electronics have batteries which are way too small for them because people would not buy things with big batteries. My digital camera has batteries that are way too small for it so I just carry a pack and plug it in.

Yes it would work but i’d go w/ the D size.

Aren’t many D cells just Cs in a larger casing? At least, so I’ve heard. And I’ve certainly never noticed that they’re more common.

Chronos, you are right that I have seen D size Nicams which were really Cs in Ds clothing (specifically at Radio Shack). They can be easily detected by capacity and weight. But true Ds are generally a much better deal than Cs, especially in alkalines.

Sources like the following do quote higher amp-hour capacities for D versus C cells of different types:

http://www.oreilly.com/reference/dictionary/terms/B/Batteries.htm

I leave it to the reader to price batteries and calculate cost per amp hour.

Many are, but some aren’t. See http://www.mahaenergy.co.uk/batteries1.shtml

  • the D cells here have nearly twice the capacity of the C cells mentioned! Note, though, that they are likely also twice as massive. If weight reduction is the goal, Cs may be better since they more closely match his existing (satisfactory) capacity.

Nerv, don’t undervolt your amp - I’d go with the full 10 cells. Note that NiMH cells will give out 1.2 volts solid until they are discharged, so you will have less warning than with the SLAs. Use a timer to keep track of your usage and make sure you don’t go over.

Also, when running large numbers of cells in series, it is possible that you will discharge one cell completely before the other cells have discharged. In this case, the voltage on that cell may go to zero and then you are running current through the cell, damaging it. Considering wiring diodes in parallel with the cells such that no single cell will discharge to less than .6 volts. The diode will allow current to bypass the discharged cell, sparing it damage.

Cheapo circuit diagram, replace each cell with a unit that looks like this:



+--------+
|        |
|        |
|        _
-        -
^diode   _cell
|        -
|        |
+--------+ 


My practice and advice is to size batteries so they can power a device for 20h at full load. Then you are using batteries at their best rate and they can handle it easily. If this is too massive, then you have to start cutting down on size but a battery with a nominal capacity of 3 or 5h is going to give you much less and will soon develop problems. Obviously, sometimes bigger batteries are not an option.

As an example with alkalines: I have a Fuji camera which uses 4 x AAs for 6 volts. I measured the max consumption (screen on) and it is somewhat over 600 mA. AA alkalines are rated at 2.85 Ah, which would mean 5 h but they do not even come close to that because the internal resistance soon becomes too high. So you are not even getting the rated 2.85 Ah out of the AA battery. But with a pack of 4 Ds (rated at 15 Ah) I would get, nominally 25 h and, in fact, I am probably getting more. Obviously I do not need the number of hours but it just makes battery management easier and you get more of the available power out. In my case I figure: AAs, cost: $3.50, last 2.75 hr, Ds cost $6.00, last 27.5 hrs, cost ratio: 5.8. So I wear a battery pack on a belt and keep AAs in the camera for a quick snapshot where it is not worth plugging in.

With rechargeables you have the same phenomenon because the faster you discharge them the less they will last and the less power you are getting out of them in each cycle.

BTW, rechargeables hold about half the charge of alkalines. It makes economic sense to use rechargeables in devices you use all the time but devices you use not very often it may make more sense to use alkalines.