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Replacing a Battery

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DirtyLude

Well-Known Member
I have a 40gb FlashTrax. It's basically a portable hardrive that I can backup a compact flash card on.

Looks like this:
https://www.dpreview.com/news/0303/03030217smartdiskflashtrax.asp

I got it really cheap as a refurbished item, and it's been working well for me. The only problem is the battery life is very short. I can backup my 512MB flash card, maybe 4 times before the battery dies. Since I will be going for a 9 day trip where I will have no access to recharging it, I need to get much more life out of it.

A new battery costs a crazy $70.00. It's worth it to me to try and make a new battery pack myself.

I've tested the battery and it puts out 3.79volts. I have a LM317T setup here that I have configured to output the same voltage. I figure I can just connect a 6pack of rechargeable NIMH AA batteries to it, regulate the voltage down to 3.79v with the LM317T, and it should work fine.

Here's what I'm worried about. I took the battery for the flashtrax out of its plastic holder and it has it's own components right on the battery. Also the components are upside down so I can't get any part numbers.

I would like to know if these are important at all.

My main guess is that it's probably part of the charging circuit. one IC is probably a temp sensor used to measure temp rise of the battery. That would explain why it's mashed directly on to the battery.

Anyway, if anyone knows what these components mounted on the battery itself, do, I would like to know, so I can figure out whether I need to reproduce these in my own replacement battery. If it's part of the charging circuit, it would be fine, since I won't be charging my battery pack using the flashtrax itself.

Pictures:
https://www.higginstribe.com/gallery/flashtrax-battery/

Sorry for the novel.
 

audioguru

Well-Known Member
Most Helpful Member
The original battery is rechargable Lithium, that needs all kinds of sensors and circuits to prevent under-voltage or over-charging. The capacity of a rechargable Lithium battery is less each time it is discharged and recharged so yours is probably worn out.
If they last long enough, 6 AA Ni-MH cells feeding an LM317T will be fine.
 

awright

Member
Yes, you could use a battery stack and LM317T regulator, but it would not be a very efficient use of the energy available from the cells. The voltage lost across the LM317T is just wasted energy unless you simply must regulate the voltage. As I read the spec sheet, the LM317T input-output voltage differential is a minimum of 1.5 volts at room temperature, and that is at only 20 ma. load current. Minimum differential voltage is higher at higher currents. You would need at least one and probably two cells just to provide this differential voltage, and that's just wasted energy. At the very least, select an low dropout voltage chip so you only need one extra cell. But I don't think you need a regulator at all. You might want to contact the manufacturer of your equipment or do a little experimentation to determine acceptable supply voltage range.

You don't mention how many cells you plan to put in series to feed the LM317T. Apparently your present battery pack puts out the appropriate voltage without a regulator. Three freshly charged NiMH cells in series will put out about 3.86 volts (measured just now on a battery pack I charged a half day ago), and I believe that their discharge curve is fairly flat (i.e., fairly constant voltage until almost exhausted). It would be an unusually sensitive device that could not handle a 2% overvoltage relative to its normal battery pack. By the way, what was the charge condition when you measured the 3.79 volts? Fresh? Idle for a long time? Partially discharged?

I believe that, unfortunately, NiMH cells have a fairly high self-discharge rate relative to other technologies. However, I don't think that would be a major problem for a trip of a couple of weeks.

If you don't have a handier source, try AA Portable Power (support@aaportablepower.com), in Richmond, CA. I have been very satisfied with their NiMH cells which I used to build up replacement battery packs for my instrumentation. They seem to stock all conceivable cell configurations.

Good luck

awright
 

Oznog

Active Member
awright said:
As I read the spec sheet, the LM317T input-output voltage differential is a minimum of 1.5 volts at room temperature, and that is at only 20 ma. load current. Minimum differential voltage is higher at higher currents.

Oh that's the least of his problems. His NiMH 6-pack makes 7.2v. If he's trying to regulate down to a lithium's 3.7v nominal, there's a 3.5v span here. That's 3.5 watts per amp, which is a lot of dissipation to use with an LM317T assuming a TO-220 pkg. In room temp free air it will cook itself at little over 2 watts. I'd personally want to keep the average dissipation well under a watt or there's heatsinking, airflow, and not burning the crap out of yourself issues here. Check your spec sheet on max pkg dissipation, also max current- that drive might take some pretty high surge current at times and you can't exceed the reg's abilities.
 

DirtyLude

Well-Known Member
Thanks for all the comments.

I have 2100mah AAs. I'm going to give a 3 pack series a try without the regulator. If it works, I might swap over to larger batteries like C cells to try and increase the lifespan... or just make up a second battery pack.

The OEM battery isn't dying, it's just too small for the gadget. It's a design flaw.
 

audioguru

Well-Known Member
Most Helpful Member
Hi Mark,
Three AA rechargable Ni-MH cells are a good match to a single Lithium-ion cell:
 

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