Converting 14.4v nimh to 7.2v

[andyeez]

Is it possible to convert a 14.4v nimh battery to 7.2v whilst still being able to use my charger which is designed for 12-18v batteries.

It is to power a R2 cree drop-in which will get very hot at 14.4v.

I will be using plenty of heatsinking + fins etc. but would like to avoid it burning out.

I am fairly new to this so in as simple terms as possible please.

 

[herulach]

Short answer is yes.

Its a bit of a faff though. If you tear the pack apart, you'll see its made of 12 cells. You'll have to disconnect into two strings of 6 and then wire the power connector in such a way that you can connect it back up in series to charge.

It'll depend what kind of connectors you're using, but assuming they're two pole, probably the easiest is to connect each string to its own socket. For power simply connect two plugs in parallel to your lamp. For chargin connect one of the plugs, then short the other two.

That should be a lot easier to understand with some wires laid out in front of you!

 

[BrianMc]

I am a new member and have been reading these Forums since July. Searches for my battery issue returned no hits. Registered and cleared to post and this thread was here!
Cue Twilight Zone music.

I ask Andyeez permission to 'double up' in his thread, as this is another battery issue with some similarity and it just makes sense for it to be here.

I have an orphaned Cygolite Z Force NiMH water bottle pack that was originally 12 volt 5 amp hr (10 cells, obviously). It has a power on LED and a voltage warning light and a PWM module that likely includes a voltage regulator to avoid over-voltage to the ballast. According to Cygolite's support, it likely has two bad cells, as it ran the HID 80% of listed runtime after careful rejuvenation over two weeks (up from 60% out of the box a bit over a year ago). So it is now a 4 amp-hr battery.

My intention: a new DIY XP-G light system using this battery for road use. NiMH seems a more stable, (if less dense) storage chemistry than Li-ion, and it's what I have for little outlay and cash is an issue. I want to have a safe and reliable battery and weight is not a major concern within reason, though this battery is pushing reasonableness in the weight department.

Question 1: Am I better off to ignore the remaining life in this battery, the energy used to make it, its carbon footprint, etc., and discard it to start with a clean sheet for my power supply? If so, why?

Question 2: If the bad cells were left in and the battery is used as a 4 amp-hr 12 volt nominal power source is there an issue?

Question 3: If leaving the bad cells is an issue, or it would be best to replace them, are there tricks or problems in identifying and replacing the bad cells?

Question 4: If mixing in new cells is a problem, then what about two 6-volt packs of 5 cells one of good old cells, one of new ones? Then what if the two sets are hooked back in series for charging with the orignal chargers? Does a difference in remaining charge cause a problem if one set is still mostly charged, or do they quickly equilibrate?

Question 5: Other than people using Georges80 Taskleds that have battery monitoring (nice), how do those running DIY lights driven by simpler buckpucks or DX drivers keep from damaging their batteries? Is it by simply monitoring runtimes as I was doing, but with no warning light to tell them it's about to get quite dark?

 

[herulach]

I am a new member and have been reading these Forums since July. Searches for my battery issue returned no hits. Registered and cleared to post and this thread was here!
Cue Twilight Zone music.

I ask Andyeez permission to 'double up' in his thread, as this is another battery issue with some similarity and it just makes sense for it to be here.

I have an orphaned Cygolite Z Force NiMH water bottle pack that was originally 12 volt 5 amp hr (10 cells, obviously). It has a power on LED and a voltage warning light and a PWM module that likely includes a voltage regulator to avoid over-voltage to the ballast. According to Cygolite's support, it likely has two bad cells, as it ran the HID 80% of listed runtime after careful rejuvenation over two weeks (up from 60% out of the box a bit over a year ago). So it is now a 4 amp-hr battery.

My intention: a new DIY XP-G light system using this battery for road use. NiMH seems a more stable, (if less dense) storage chemistry than Li-ion, and it's what I have for little outlay and cash is an issue. I want to have a safe and reliable battery and weight is not a major concern within reason, though this battery is pushing reasonableness in the weight department.

Question 1: Am I better off to ignore the remaining life in this battery, the energy used to make it, its carbon footprint, etc., and discard it to start with a clean sheet for my power supply? If so, why?

See Q2

Question 2: If the bad cells were left in and the battery is used as a 4 amp-hr 12 volt nominal power source is there an issue?

You likely haven't got two bad cells (which would probably wind up dropping the voltage, stick a multimeter over it, if it reads less than 14V or so straight off the charger then it is a bad cell, in which case either sling it, or remove them, but you'll need a new charger
As you use them, particularly to full discharge, NiMh batteries will have less capacity.

As a completely unrelated aside, if your battery isn't made of 20 cells (I'm assuming they're subCs its nowhere near 5Ah anyway

Question 3: If leaving the bad cells is an issue, or it would be best to replace them, are there tricks or problems in identifying and replacing the bad cells?

See above, but don't swap cells to new ones. Take them out and leave them out or sling the whole pack.

Question 4: If mixing in new cells is a problem, then what about two 6-volt packs of 5 cells one of good old cells, one of new ones? Then what if the two sets are hooked back in series for charging with the orignal chargers? Does a difference in remaining charge cause a problem if one set is still mostly charged, or do they quickly equilibrate?

Short answer is yes, you'll either not charge the discharged one or overcharge the charged one depending how your charger acts.

Question 5: Other than people using Georges80 Taskleds that have battery monitoring (nice), how do those running DIY lights driven by simpler buckpucks or DX drivers keep from damaging their batteries? Is it by simply monitoring runtimes as I was doing, but with no warning light to tell them it's about to get quite dark?

I use a DX LiPo display, this is basically a little voltmeter on a board that flashes up the voltage. Wired up to the same connector as the light takes about 30 seconds to check. Failing that, using a Voltage comparator IC to make a little status LED shouldn't be the most difficult thing in the world.

 

[BrianMc]

Thanks. That is a big help. Can check voltage. Can add the monitoring.

Can live with it at current max storage for now. Tried to always shut it down with the red LED. Only about 20 minutes left in it at that point the one time I had to push it.

Cygolite lists the HID it drove as 14 watt. The also listed the battery as 5 amp hour, though. Estimates, guesstimates. Likely uses at least another watt in the voltage regulator and ballast for at least 15 watts. Light took 3.3 hours from full charge to red LED or about 50 watt hours. So about 4 amp-hours from full to red LED after sitting on the shelf for 2-3 years and used for one. Maybe they just got 5 amp-hr from full to full discharge when brand new on a nice warm evening. Several posts existed a year ago that indicated few ever saw 4 hours of run time even when new, more like 3.6-3.75 hours, if I remember correctly.

 

[BrianMc]

See Q2
You likely haven't got two bad cells (which would probably wind up dropping the voltage, stick a multimeter over it, if it reads less than 14V or so straight off the charger then it is a bad cell, in which case either sling it, or remove them, but you'll need a new charger

Out with the very old VOM and in with the new VOM to test this nominal 12-volt NiMH pack.

Cygolite provided both a "smart" fast charger which tapers off the charge as the battery reaches full charge and 'tops off' periodically and a 12 volt wall-wart slow charger.

Voltage (fast charger showed a green led 'fully charged' for 7+ hours): 13.88 volts.

Two, almost new AA NiMH cells hot off a charger, within minutes later:
1.36 and 1.38 volt each.

So 13.88 for a 10s2p pack looks like a full charge with no bad cells.

Popped the top and the pack is very nicely sealed up. It would be best to run it into the ground as a 12 volt supply. Electronics and connections are pristine.
So I can save a few bucks here for a while.
The 13.9 volts also means that there is no voltage regulation. Just circuitry for ramping up the HID. Your monitor will work like a charm.


Much obliged Herulach. Where's the "I love CPF" smiley when I need it?

 

[Christexan]

Just to expand a little bit, a 10s/2P pack is a 12V nominal pack, not a 14.4 (at FULL charge hot off, brand new, it'll hit 14.4 for a few seconds, at which point under load it'll fall to the 12V range (typically a little over) within 10% of the total runtime (if it last 3 hours, it'll drop to nominal after 0.3 hours, depending on load, as an example)).

So, ultimately, it sounds like your pack is working great actually. There is likely a boost regulator as part of the HID bulb (maybe in the ballast, or elsewhere in the lighthead) that is stepping up the 12V to the 14.x needed for the bulb. What it means is you ARE getting your runtime, the amp-hours doesn't mean the runtime in hours. I don't know your HID wattage, if it's a 10-watt, at 14V (at the bulb), it means it needs roughly 1.4A of current to operate. Your pack is providing 12V, so you have to convert it (before losses) by stepping the voltage up from 12V to 14V (1.66...) which means the current drawn from the battery pack has to increase proportionally, so the 1.4A at the bulb = 1.4*1.67 = 1.63amps of current draw from a 12V pack

Pack is a 5Ah pack, drawing 1.6A = 5/1.6=3.125 hours of runtime. At the initial full charge, the pack has a higher voltage so the draw is less, so figure an extra 10%, so 3.4 hours of runtime. Which nearly exactly matches what you are seeing (I think, from all your posts).

That is actually exceptional if you are seeing 3.3 hours of runtime, it means the efficiency from battery to bulb is VERY high.

If however you have a 12-cell 14.4V nominal pack (6s2p), then that is "typical" runtime, and still means you have a good pack. Either way, a several year old 5Ah rated NiMH pack providing a 10W/14V HID bulb with over 3 hours of runtime is a stellar pack. Keep it until it dies!

Maybe I misread something, but that is my take on it.

 

[herulach]

No bother - the display on the meter flashes on and off, so it would be distracting to have on all the time. Might be as well to slot it down inside the top of the pack.

See Q2
You likely haven't got two bad cells (which would probably wind up dropping the voltage, stick a multimeter over it, if it reads less than 14V or so straight off the charger then it is a bad cell, in which case either sling it, or remove them, but you'll need a new charger

Out with the very old VOM and in with the new VOM to test this nominal 12-volt NiMH pack.

Cygolite provided both a "smart" fast charger which tapers off the charge as the battery reaches full charge and 'tops off' periodically and a 12 volt wall-wart slow charger.

Voltage (fast charger showed a green led 'fully charged' for 7+ hours): 13.88 volts.

Two, almost new AA NiMH cells hot off a charger, within minutes later:
1.36 and 1.38 volt each.

So 13.88 for a 10s2p pack looks like a full charge with no bad cells.

Popped the top and the pack is very nicely sealed up. It would be best to run it into the ground as a 12 volt supply. Electronics and connections are pristine.
So I can save a few bucks here for a while.
The 13.9 volts also means that there is no voltage regulation. Just circuitry for ramping up the HID. Your monitor will work like a charm.


Much obliged Herulach. Where's the "I love CPF" smiley when I need it?

 

[BrianMc]

Just to expand a little bit, a 10s/2P pack is a 12V nominal pack, not a 14.4 (at FULL charge hot off, brand new, it'll hit 14.4 for a few seconds, at which point under load it'll fall to the 12V range (typically a little over) within 10% of the total runtime (if it last 3 hours, it'll drop to nominal after 0.3 hours, depending on load, as an example)).

Not sure, but I think there is a confusoin about the original poster with a 14.4 volt pack (hence the title of the thread and my horning in question about a 12 volt nominal pack (labeled so on its back), since it seemed related. No problem.

So, ultimately, it sounds like your pack is working great actually. There is likely a boost regulator as part of the HID bulb (maybe in the ballast, or elsewhere in the lighthead) that is stepping up the 12V to the 14.x needed for the bulb. What it means is you ARE getting your runtime, the amp-hours doesn't mean the runtime in hours. I don't know your HID wattage, if it's a 10-watt, at 14V (at the bulb), it means it needs roughly 1.4A of current to operate.

It was a 14 watt bulb so 40% higher. There appear to be both 14 and 12 volt HID bulbs. I have no idea whether this was a 12 or had to be stepped up. But...

Your pack is providing 12V, so you have to convert it (before losses) by stepping the voltage up from 12V to 14V (1.66...) which means the current drawn from the battery pack has to increase proportionally, so the 1.4A at the bulb = 1.4*1.67 = 1.63amps of current draw from a 12V pack.

If it was a 12 volt 14 watt bulb and not a 10 watt 14 volt, then the runtime comes ou the same, right?

Pack is a 5Ah pack, drawing 1.6A = 5/1.6=3.125 hours of runtime. At the initial full charge, the pack has a higher voltage so the draw is less, so figure an extra 10%, so 3.4 hours of runtime. Which nearly exactly matches what you are seeing (I think, from all your posts).

What I WAS seeing, yes.

That is actually exceptional if you are seeing 3.3 hours of runtime, it means the efficiency from battery to bulb is VERY high.

If however you have a 12-cell 14.4V nominal pack (6s2p), then that is "typical" runtime, and still means you have a good pack. Either way, a several year old 5Ah rated NiMH pack providing a 10W/14V HID bulb with over 3 hours of runtime is a stellar pack. Keep it until it dies!

Maybe I misread something, but that is my take on it.

It matches up nicely. Cygolite should have claimed 3 hours run time and everyone would have been ecstatic! Maybe it was a misprint.

Thank you. $70-100 in a DIY Multi XP-G headlight, nice not to have to buy a battery, too.

I have been checking its voltage running an LED MR16 10 degree circa 80 lumen spot light (I am seen and can see if I keep it below 18 mph, or so and concentrate. Runtime about 660 minutes compared to 200 for the HID (500 liumens). (Don't have to aim it into the ditch to keep from blinding motorists, but I need to see better.)

Start 13.8 volt, after 50 minutes ~8% discharged: 13.6 volt, after another 110 minutes, ~24% discharged: 12.6 volt.

Stil not down to the 12 volt plateau after 24% capacity has been used. So I agree with you, this looks very good.

It is as Herulach suggested: a bit of Multimeter work and all is revealed. I just needed a little support/shove to jump in.

 

[Christexan]

I think I actually got 2 totally different threads confused, on a thread somewhere I saw a mention of a "14V" stamped on an HID ballast, and it was probably a completely different thread (or even forum, LOL, may have been MTBR) (a problem I have of replying to posts during the day, at work, distracted, and squeezing in bits and pieces of a reply throughout the day as time allows, LOL, sorry about that).

But yes, if it's a 12V/14watt spec bulb and 12V pack, the "brand-new" Ah rating should roughly match the amp-hours drawn by the bulb (watts/volts=amps so 14/12=1.17 amps, minus 10-20% for efficiency losses and "overrated" MFG specs, etc). With good quality NiMH batteries, and good maintenance (charging and/or using/recharging at least once every month to 6 weeks instead of letting the pack sit for months at a time), you can expect to lose 10-20% capacity per year. So if after a couple of years, you are still getting 80% or more of the rated Ah in terms of runtime, that's a really good pack.

Using your (BrianMC) <rereading the thread, LOL, to avoid confusing my reply> original 5Ah pack, you are drawing roughly 1.17amps. That should equate to a "brand-new" ideal battery runtime around 5/1.17=4.27 hours.

Taking out system efficiency losses of at least 10%, and a conservative 20% aging losses (10%/yr typical minimum), and you would expect roughly 3 hours of runtime, with all cells still in "good" condition considering their age.

So after all that and the confusion on my part, it still sounds like a pretty good pack to me. Although the cells in theory can hit 1.4xV/cell hot off the charger (14 volts total pack), that's when new and all cells are balanced in their SOC (all at 1.4x volts).