Digital Camera Battery Pack Issues

***Note if you're looking to read this just to make your own battery pack, you may want to skip to the bottom, where I offer advice after having figured a bunch of stuff out.***

Hi battery experts out there,

I'm confused. I bought five Tenergy D size NiMh batteries (not the premium ones but the blue and cyan ones, which I'm now regretting) a couple months back. I'm trying to use them to power a digital camera that typically runs on four AA alkalines. It isn't working, and I want to know why. The camera powers on and does everything it should do, but when I hit the shutter, it powers off and restarts.

My suspicion is that this has to do with NiMh batteries having a lower nominal voltage and larger voltage droop under high current drain, but I would have thought that the extra battery would have taken care of the first problem and the large size would have taken care of the second.

What am I missing? Is this an issue I can solve with a capacitor or some little chip off ebay that would store up a bit of energy and release it over the half-second picture-taking event? Where else might I ask about this?

Additional info possibly related:

The voltage reading for the five batteries in series when recharged (using the Tenergy Speedy Box after a few charge discharge cycles) is 6.72V (I think this is very close to four fresh alkalines, right?).

The camera can supposedly be run off a 4.8volt, 2.4 amp adapter (I don't have one).

I've a dummy battery setup to plug the D's into the same circuit the AA's would be in. I'm fairly confident that the problem isn't the wiring of this setup.

I've tried plugging the batteries into the DC adapter port; the results are the same.

With four NiMh batteries, I get a battery empty message from the camera, so I don't think the issue is that I'm over powering it.

If I switch out one or two of the NiMh's for alkalines, the camera will take some pictures.

Thank you so much for your time,

Adam

Original Post from 10/10/2011 Updated 10/12/2011

Well, I did some reading.

As it turns out, the size of a NiMh battery isn't nearly as directly related to its voltage drop under load as I had hoped (the size is more a determinant of how low this drop could be, but it's cheaper to manufacture the batteries if they have a fairly large drop, so you've got to be sure you're getting a high quality, "low internal resistance" battery if you want to use it in a high current draw application).

What is more related to the current drop of the battery is its chemistry; while the new NiMh batteries are good for their capacity, low cost, and reliable charging regardless of their amount of discharge, the older NiCd's remain better in terms of weight and low current drop under load (If I was going to make this battery pack again, I'm thinking I'd try them instead. As it stands, I'm too invested in my NiMh's).

Because this reading suggests strongly that the problem is voltage drop under the heavy current load (if so, the batteries must be dropping to just under 1V each under a near 2.4Amp draw) I looked around for a device that can boost my voltage a bit.

So far as I can tell, the best way to do this is with a DC-DC boost/buck converter. This is a device that takes a variable voltage in and spits out a constant one that you can generally adjust. Of course, the thing isn't providing free energy, it's going to eat up some power doing the conversion and thus the current draw will now be even more than what it would have been, but for my 10Amp hour batteries, this should be just fine, I think I'll still be well under the point at which they'd be depleted in 2 hours, which (I've heard) you generally don't want to pass with rechargeable D cells.

The converter is in the mail, and I'll post the results when I get them, but I'd be interested to know if anyone has experience with such devices. My electronics knowledge is (obviously) pretty limited, and I was particularly worried when I saw that many of the converters are labeled "constant output current". The idea that a device can supply a constant output current regardless of what it's connected to doesn't quite square with my understanding of electronics (I thought the current had to depend on the resistance of the load, which can always be varied). If the converter is going to supply a constant current, I'll have to set this current to the maximum required current of the camera, and then I worry that 1) I'm depleting my batteries unnecessarily fast, and 2) I'm going to fry the camera which isn't made to draw it's max current all the time. On the other hand, I guess a 3Amp draw (what I figure I'm going to need to send to the converter to get 5V and 2.4Amps out the other side) would still give me 2 or 3 hours of run time, and I guess the adapter the camera can run off probably also would supply a constant current (right?), so maybe it wouldn't be a big deal(?).

If anyone has knowledge of such things, I'd be super interested to hear about it.

Thanks again,

Adam

(Another addition, 1/11/2011)

Well, rats. I got the DC-DC converter, and it didn't solve the problem. However, after more reading and thinking, the clarity of what is going on is improving, I think.

The deal is this. Not only is voltage a problem, but so is current. As one increases the resistance of a circuit, the amount of current that passes through it drops. While my nifty voltage adjusting component allows me to make sure the camera gets a steady voltage, it also happens to have some resistance. In fact, it has just enough resistance that when the shutter fires and the camera lowers its resistance to ask for more current from the setup, there's still enough resistance in the circuit that the batteries can't deliver.

So what to do? I went back and forth between a few options here, and I'll indicate what I think their merits are.

Option 1: Forget the NiMh's, and buy five new NiCd's which have lower internal resistance and lower voltage drop under load.

This would have been cheap and almost certain to work (other's have done it successfully with similar cameras), but it would have meant the 50watt*hours of battery I've already paid for wouldn't be used. Additionally, it's still unclear exactly how much less voltage drop NiCd's have and how much more current they can provide, so I can't be completely sure they'll work with my camera.

Option 2: Forget the NiHh's and use cell-phone-style lithium ion batteries.

It occurred to me that this might work while rummaging angrily through my parts drawer. The flat Lithium Ion cells one gets for cell phones are cheap, quick to charge, long lasting, and charge to around 3.8 volts, so two of them would be great for my camera. I tried this and it did work. Ultimately though, I don't have a smart charger that I takes Li-Ions and I've only got two spare cells (each 3.7 V, 920 mAh), so this isn't going to provide me the long-lasting solution I want without more investment, and again, wasting the NiMh purchase.

Option 3: Buy a DC-DC converter with adjustable resistance (read current supplied) as well as adjustable voltage conversion.

This would have been a cheap solution with minimal waste (I wouldn't use the DC-DC converter I already bought, but I would use the batteries). However, I couldn't seem to find a way to be certain of whether or not the DC-DC converters I was looking at on ebay would be able to actually drop their resistance low enough that they'd pass enough current to my camera. Furthermore, the converters with both variables are a bit pricey.

Option 4: Buy more NiMh batteries.

This is what I did, though I'm not yet sure it will work (or that it even works in theory). My thinking is that I can either hook another set of NiMh cells up to the ones I've got and wire them directly into the camera, which should draw only half the current from each set and thus reduce the voltage drop to a negligible level, or that I can wire them all in series, and plug the 10V or whatever into my DC-DC converter, which because it more or less conserves power (Power = Current x Voltage) should be able to kick out more current because of the higher voltage. The benefit of this plan is that it wastes nothing I've already bought and should give me a 100 watt*hour pack that'll last for dozens of hours per charge. I did buy the slightly more expensive high-discharge version of the D's this time, so maybe if nothing else, I can just use them instead, plugged into the camera directly.

***Note***

If I was going to do this all again, I'd do it differently. I'd probably buy two decent flat-style Li-Ion cells and a compatible charger (all-battery has many to chose from, the 3.7V 5Amp ones seem a decent deal, else one might try four or six ebay cell phone batteries with two groups in parallel) and just plug them right into the camera's DC in port via a dc connector and a bit of soldering or electrical taping. If not that, I'd just buy five D or F sized NiCd's and see if that worked. Only in the case I really needed some serious long life would I go the 10 NiMh cells approach, and I'm still not sure it's going to work without a more expensive DC-DC converter.

***End Note***

I'll post one more time when I finally get this rig working. Again, if anyone has any insights as to what I'm mucking up or getting right, that information would be super useful.

Slowly learning,

Adam

(Final addition, 11/05/2011)

The second set of batteries arrived in the mail, and with them, my little DC-DC converter happily puts out up to 2.9amps or so, more than enough to run the camera. I also realized at some point that the cheapo alligator-clip leads I was using were getting hot (and thus wasting a bunch energy), and replacing them with some thicker wire and solder rather than clips also made a big difference as far as lowering the resistance of the circuit (and thus increasing the current).

Battery pack complete and working,

Happily Photographing,

Adam

If anyone has questions about any of this stuff, I think replies will show up in my email, so don't be put off if you find this a while from now and think I might be able to help out.