Will this series work?

[Juggernaut]

Ok, in my never ending quest to build the cheapest, 4557 bulb running light. I just bought 2x 13,000 mah NiMH F sized cells to run in series with my 2x 18 ah SLA batteries. I figure with the SLAs being old and only giving that 18 ah rating at 20 hours or what ever that the two battery types should have about equal capacity of around 13 ah. Any who, what I was wondering is whether there is any specific order to assemble these cells? I was going to have a SLA + to wire, - to F cell, that F cell would be attached to the other and then that one would connect with the last SLA. So basically the two NiMH cells will be between the SLAs. Is that ok? Should I set it up differently:thinking:?

I know the picture is small but that's the best I could get, the blue cells are the F and the black are the SLA, with the bulb, gray. Green is the wire.

 

[Mr Happy]

It doesn't make any difference what order you connect them in -- the electrical result is the same with any ordering.

However, make sure you stop discharging before any of the batteries run down. If any battery gets near empty before the others it will be damaged. And make sure you charge each battery separately before connecting them.

You doubtless know that what you plan is a bad idea, right?

 

[Juggernaut]

This should give me a good amount of run time, I am only asking from the batteries the equivalent of a 12 volt, 55 watt bulb running on a 4ah SLA "which there are lights on the market like this" mathematically I should get around 30 minuets of run time, however as you stated this really, really doesn't sound to, well sound! So I'll only use it for like 5 minuets and then it will be back to the wall to fully charge all the batteries for the next expedition:ironic:. I have ran the light off of two of these SLA batteries. For a couple of minuets, a couple of times and they worked fine:thumbsup:, but I know I could get more power by upping the voltage to what I need:twothumbs. So I figured the two F cells will give me what I need. Next I can only charge each battery individually anyways so that shouldn't be a problem, so I know the state of each one.

Though are there any other flaws you can see in this plane? I know it's anything near ideal, but this is the best I can do for the money, oh and I won't be firing both filaments at once so there will be only a 25 amp draw (700 watts)instead of a 35 (1000 watts) to at least remotely lessen the burden on the batteries.

Thanks for the input.

 

[Norm]

You should never mix different capacity or voltage or different chemistries of batteries. All batteries in any pack should be identical.
Even two or more batteries of exactly the same type but with different states of charge are dangerous.
Not a wise idea.

 

[Juggernaut]

I said exactly this in some other thread, and someone, told me that as long as the capacities were similar, it would be ok. Which changed my mind set. If I put a 2 volt SLA rated at 18 ah in series with the other two batteries it would have no adverse affects, so I figure this would be fine using a NiMH.

Ok, after some searching this is what modocod said after I told some one not to mix different chemistries:

This is a good general rule of thumb to follow, but each case can be looked at individually and you can usually come up with exceptions to this general rule....

In this situation, I think it would be fine, if you added a 10AH D size NIMH cell in series with a 6V SLA that was only say, 4.5AH, or whatever, then I don't see a problem and here's why....

This is a direct drive light, so there is feedback in the form of output to let you know when the battery is about dead....The SLA would go dead long before the NIMH, so you would know the battery is dead as the brightness would drop way off. The NIMH cell would still be sitting with some juice left in it and would not be reverse charged by the SLA. The biggest concern for this setup, is the health of the NIMH cell that is being added to the mix. The SLA will not be effected in this configuration as it represents the bulk of the pack being put together.

The main concern about mixing different chemistry or different capacity cells, is when you have a circumstance where there would be no way to know that one of those cells in the mix has been completely discharged, like for example, a regulated HID or LED light. Or in the case of a higher voltage bulb with lots of cells in series, a single cell dropping out may not provide enough useful feedback in total output to know anything was wrong...

It would be a BAD idea to take this flashlight, and wire an extra AA NIMH cell in series with it, as the AA NIMH cell would drain down before the SLA, and if left "on" in the circuit, could be severely over-discharged, and actually reverse charged, which is the best way to ruin a NIMH cell.

In any case where you are going to have 1 "oddball" cell in the mix, it's best to have that cell have more capacity than the rest of the cells in the configuration, so that the rest of the pack goes dead, with the obvious result of the flashlight giving a useful amount of feedback to the user to shut it off, the 1 oddball cell is not damaged, and neither is the main battery pack....

The user would need to make sure to charge up that NIMH cell whenever the spotlight is charged up.

Eric

Now technically, the NiMH cells are rated at less capacity, and there are two of them, however I believe that the SLA's overstated capacity, combined with their reduction at almost 2C discharge + the fact that they are old and used "2 years" will allow them to be pretty close to the capacity of the F cells, and as I said I will make sure not to let the whole setup drain to much before recharges:thumbsup:. Theoretically the F cells should still run this load for 30 minuets regardless of what the SLAs are doing.

 

[Mr Happy]

What you are talking about here is not really dangerous in a personal safety sense, but it can easily be harmful to the batteries. If you start out with fully charged batteries, and run for only a short period while watching the voltages carefully, you could get away with it. However, it is not a practical setup for anything more than experimental purposes.

 

[Juggernaut]

Thanks for all the input:thumbsup:, I'll defiantly put all this into account, when you build a light out of stove piping, cut- extension cord, industrial fan switches, UPS batteries, lunch box straps, duck tape, zip ties, trashed picked wires, 3/8in pressure treated plywood, and UPS cardboard boxes; I can only hope I'll use it for experimental purposes:laughing:. The only two things I have concern for is the F cells, and the bulb, since everything else is disposable and free.

 

[LuxLuthor]

I think when the guideline is given not to mix cell types, we are talking about using them on an ongoing basis, including charging. They have different internal resistance values, and current flow rates/limits. This can affect temperature of cells and internal pressures. I don't know if I agree with all of Eric's (mdocod) speculations, and I would not assume that non-battery professionals/engineers have adequate knowledge to advise making exceptions to the well accepted guideline about not mixing chemistry and cell types...such as SLA & NiMH.

As Mr. Happy says, this "Frankenstein Battery Pack" is not on the same risk scale as screwing around with Lithium Cobalt, but ask Silverfox if even NiMH can explode and do damage.

 

[Norm]

I stand by what I said, to me this is very poor practice. I agree if this was a quick test set up that's on thing but this appears to be the final arrangement for the battery pack in a light.

Ok, in my never ending quest to build the cheapest, 4557 bulb running light.

.
As Lux says.

I would not assume that non-battery professionals/engineers have adequate knowledge to advise making exceptions to the well accepted guideline about not mixing chemistry and cell types...such as SLA & NiMH.