Question about battery pack sizing

Good Day

I have a question about batteries for a bike light. I have build a bike light using a MR16 bulb of 20 watt size. I like the light output and beam shape and amount of spill. My question is whether or not I am drawing to much power from the battery pack and thusly damaging it. The pack is made of 12 AA rechargeable 2500mah NiMH batteries. Initial voltage is 16.1 V and drops to 13 and falling after turn on. After a 15 minute ride the voltage has stabilized at 11.9 V. After turn off voltage returns to 15.8 V. Lots of light output. Am I overdriving the batteries or is this about normal. If needed I could get a 10 watt bulb.
Thank you for your answers.
Ian in Kansas

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Hopefully some one will have an idea.
Thank you
Ian

yes it is running them hard, your pulling about 1C on the batteries, that is not terrible, what will be terrible is if they get reverse charged (one goes to 0) at that load.
if it is a 12Volt bulb, and not a 12V bulb designed for automotive use (14.5v actual) then your overdriving the bulb too.

12 not 10 ni-mhy cells would pump a good 14volts out. which could be nessisary with long wires or regulation, but its being mean to both parts if its a "true" 12v bulb and you have 12nimhys on it.

so you might be using even more than 1C out of the batteries in the first 15 minutes or so. that is dependant on the resistance in the whole thing, like the wires and all. and you might be running more than 20Watts.

Sounds like you have a nice hotwire there
did you do an AMPS check to see what the actual current is when the light is on?

myself i might have gone with 10X nimhy or 11 if the load dropped it to much, but 12 sounds like your pushing it up to hotwire status.

should you change to a 10W bulb? well at the voltage you might pop a 10W bulb even faster? depending again on the actual voltage of the bulb. your high voltage ni-mhy could snap it faster still.

is there any regulation?

you could lower your input voltage (less cells), just as easily as you could lower the resitance of the bulb (less power), and achieve some of the same goals.

it sounds a bit like your batteries are self regulating to the bulbs voltage , instead of the other way around.

Hotwire experts would know more about this,
me i would drop a cell AND use the 10W,
or use 10 or 11 ni-mhy Sub Cs or Cs with the 20W

I like runtime AND output, so i would try for a battery bulb combo that lasted some 2 hours at least.
we drive a 100W video light with 10X2XD cells 12 volts parellel D cells, the batteries get a bit warm, it runs 1.45hrs
although i have added a single extra cell per set , when the wire lenght was dropping a volt.

Hello Ian,

Welcome to CPF.

I think you may be driving things a little too hard, or you may have a questionable cell in your pack.

When you turn your light on, you have around 13 volts. That's about 1.08 volts per cell. A nice goal to shoot for is to have around 1.2 volts per cell under load. After 15 minutes, you are under 1 volt per cell, so your total runtime may only be slightly more than 15 minutes.

This means that you are discharging at 4C, and that is a pretty high discharge rate. At that rate, you may be able to get 40 - 75 cycles from your pack.

On the other hand, if you have a bad cell in your pack, you may be getting low voltages due to a bad cell instead of a high load.

Keep in mind that there have been a lot of "issues" with all of the 2500 mAh cells.

Have you verified your current draw?

If you review the NiMh shoot out thread you can see how different cells behave under various loads. Most of the cells stay above 1.1 volts at around 5 amps. I believe this works out to around 66 watts. If your pack is dropping to below 12 volts at 20 watts, something is wrong.

Tom

Hi VidPro

Thank you for the reply. To answer some of your observations, here is a list of some of the design guidelines I am using. For a halogen bike light it is common to overvolt the light by about 20 percent. For example use a 10 volt bulb with a 12 volt battery or a 12 volt bulb with a 14.4 volt battery. At least that is the theory I read over and over on the web. That is why I used 12 1.2 volt NiMH batteries (total of 14.4 volt). Just before I tried this battery pack out I read that NiMH batteries come from the charger with a high voltage (about 1.5 to 1.6) and that you should let them set for 24 hours, then they will drop down to about 1.4v. I did seem to experience this. Then the batteries should drop down to 1.2 to 1.1 in use, at least this is what I have been reading on the net. My starting voltage after a one day rest period of 16.1v would seem to agree with this. This is definitely more over voltage than I was intending but with out trying the light I could not predict what was gong to happen. I probably should remove one cell. My biggest concern is that the total voltage drops to 11.9 volts under load, and is well under the desired 14.4 volts, You can see the light dim a bit in the first five minuets of use. Going to a 10 watt light should cut the current in half and result in less voltage drop. I will just half to try it and see just how much.
One of the problems I have run in to is the lack of information from the manufactures on maximum current draw from there batteries. For example Energizer states that there AA batteries are rated at 2500 mAh at 500mA but they don't give a maximum current usage. They do have graphs showing voltage verses current verses time for different loads, with the highest load being 5000mA. The curve for 2500 mA shows about what I expected for run time but a higher voltage than I am experiencing. They show about 1.2 falling to 1.1 volt. I wander if there is enough resistance in the battery holders and there wiring to be causing some of this problem? I guess I should measure an individual cell under load as well as the entire pack. I will try that tomorrow. This is all a learning experience.
Ian

Hi Tom

As I was writing my reply to VidPro I was beginning to get the feeling that there might be a problem with the batteries, maybe a bad cell or a high resistance in one of the holders. I don't like the holders (Radio shack) and will definitely look at some alternatives. I will test all batteries for voltage tomorrow with the light on. Since 20 watt at 12 volt is 1.66 amp and at 14.4 volt is 1.38 amp there seems to be something wrong. Will trouble shoot tomorrow. Thank you for the input.
Ian

yes depending on the battery holders it can be a lot of resistance.
then if the cyclist has it back in the water bottle holder, there is more wire resistance. that could be why they recommend the 2 extra cells instead of 1.
this also would vary by large quantites depending on the wire used, and its length .

each sitation of assembley would differ slightly from user to user, in ways that can be quite a bit different.

but Silverfox knows hotwires, sooo, he can help you much more.

when we have to use ratshack holders we have to replace all the wiring on the holder, and cross solder the riveted springs to the spring itself, because the rivet compression is not sufficent connection.
but that of course is on the c and d holders. also the wire is complete junk and is replaced.

Some cells handle that kind of current better than others, generally speaking, that is not a hard duty for a decent NIMH cell in that size class... If the cells are performing correctly, you should be getting over an hour of runtime from that, so it sounds like something isn't working right there... FYI at 14.4V a "12V" rated 20W halogen will become a 26.6W bulb pulling about 1.85 amp, which is very reasonable current IMO, but again, it depends on the cell. most off the shelf flashlights that run alkaline draw about 0.7A, most decent NIMH cells in the AA format seem to do fine up to around 2A, some cells seem to tolerate 5+ amp just fine, special hobby style cells (sold as high current cells) can actually deliver 20-30 amps without breaking a sweat...

Also, I personally am of the belief that 12 cells is absolutely FINE if you are dealing with a typical off the shelf track lighting bulb rated for 2000-5000 hours (most are in that range)... I would suggest picking out the bulbs rated 2000 hours, as they will be brighter then the 3000+ rated bulbs. Even at 14.4V, a 2000 hour 12V halogen should last over 200 hours... for the purpose of efficiency balanced with reliability I would recommend shooting for 30-50 hours of re-rated life. I have run 20W MR-16s on 14 AA cells with no insta-flash problems, and I believe you could even go as high as 15 or 16 cells without insta-flashlight bulbs on a regular basis if you are dealing with bulbs rated for over 2000 hours at 12V... For your application I'd suggest 13-14 cells to make the bulb run more efficiently.

As for what type of cells to run.... AAs are fine IF they will provide the runtime you need (should get about an hour on decent cells) and are of good enough quality to last enough cycles to be worth it.... but consider slightly larger cells of you want to get longer rides in.. Sub-C cells are very energy dense per volume and weight as they are very common in the RC crowd and see a lot of R&D to push the limits, 4500mAH with 40A capable drain rates is very common in those cells. For these hobby cells a 12-13 cell pack would be my recommendation as they tend to hold voltage up pretty high under a load. 2 6 cell flat packs (pre-made) or a 6 and a 7 cell would be easy to wire up in series with a few plugs and wires, and easy to charge on common hobby pack chargers, the fact that the pack would be all soldered up would also help reduce resistance...

I'm curious to know what brand of AA you are using there, as it sounds like they are really biting the dust under what should be a fairly easy cake walk for a half decent cell..... If you decide to build yourself up a new pack I would suggest eneloops for this application as they hold up very well (1.2V+) through most of a discharge at 2A. Not to mention they will probably have a longer service life, they just seem to be more robust overall and therefor easier to maintain and deal with.

Hello Mdocod,

There has been some discussion of the higher capacity Sub C cells (4200 and 4500) developing the same rapid self discharge issues that we see with the AA 2500 cells. They work great "hot off the charger," but can be completely discharged through self discharge in a couple of weeks.

3500 - 3700 seems to be the threshold in Sub C cells. However, keep in mind that the RC people tend to push cells to their limits, so it could be an extreme performance thing...

Tom

Figures, lol... (knew it had to be too good to be true)... but then again.... what do RC people care about self discharge, lol... I used to be into that pretty heavy, It was always melting the pack on the car, followed by melting the pack on the charger, if things weren't burning your hands you weren't having fun! Pack didn't sit more than a few days un-used lol.

Hi Mdocod
Thanks for pointing out that when the voltage is changed then the wattage is changed as well. I had read that but had forgotten it. The batteries are Energizers. Here is their engineering specs page

http://data.energizer.com/PDFs/nh15-2500.pdf

My actual nightly commute home is only about a mile and a half between 7 and 8 pm, Sometimes I add extra just for the fun of it. However I don't want to recharge the batteries every night so if they make it 4 or 5 days that would be great. I will sort that out once other problems are fixed. Thank you for all the information, It helps me know I am on the right path.

Hi VidPro
I think the test results tonight show that the holders and maybe the wiring are the trouble makers. Will try to rebuild them very soon and am looking for replacement ideas.

I enjoy doing for my self so this entire process is an enjoyable learning experience.


I did a lot of measuring tonight and these are the results.

No load bat voltage 15.19
No load cell voltage all were 1.27

after a 5 minuet run to stabilize things

Load bat voltage 12.1 to 11.9
Load cell voltage 1.19

The individual cell voltage seems safe
I checked all cells for polarity with the meeter and they were all correct.
The ends of the four cell holder became worm.
The connector on both cell holders became warm.
So far I am going to guess that the problem is resistance in the holders.
Currant starts at 1.62 amps and drops to 1.57 amps in two minuets.

I will let you know how I get along with this.
Thank you again.

Ian

NiMH 4/3A cells can be had at 4500 mAH. Check Batterystation.com