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Battery and Charger Strategy - any ideas?

Our product is an automotive battery tester which recharges its internal 6V SLA battery from the same clips which perform the battery test. Owners tend to clip the tester on a spare battery when driving between breakdown service calls, and it takes a "drink". Internally, we have temp-compensated voltage charging, so the SLA is charged quickly and prolonged charging only floats the battery nicely. If the SLA runs flat, you can get testing again after a few minutes clipped to a battery.

Now we have to replace the SLA with Li-Poly or NiMH or other non-lead technology, but it gets tricky.

NiMH batteries tend to be used in cycle use, where you fast charge up to a voltage inflexion or delta temperature. Trouble is, we can't initialise a fast charge every time the tester connects to a battery because 50 fast charge cycles a day will destroy the battery!

GP Batteries rate NiMH cells for 0.1C trickle charge over long times, so one option is to charge at 0.1C whenever the clips are connected. Unfortunately, this means a 16 hour charge time from flat, whereas the SLA got 90% charged in an hour or so.

Li-Ion or Li-Poly also tend to be used in cycle mode. They do voltage charge, like the SLA. But they don't like to be at full voltage indefinitely - you float for 3 hours after charge, then stop. Trouble is, when the clips are connected, you don't know if you are continuing a previous long charge, or if the battery is ready for the 3 hours. In addition, keeping a Li-Poly at full voltage tends to reduce its capacity, and mechanics tend to keep the tester on charge. I wonder if we can reduce charge voltage slightly and put up with 85% of capacity, to lessen the capacity deterioration effect when constantly under charge.

I can think of clever microprocessor schemes, such as coulomb counting, but we'll need field trials and the project is "interesting"

- ie. will have unexpected, time consuming and costly lessons to teach us. Also spark plug pulses will reset the processor at times. The coulomb counting IC typical circuits seem to show horrendous quantities of parts.

I'm not keen on Li-Ion or Li-Poly, because of the brutal handling by mechanics. Testers reach 60C sitting under the windscreen, get driven over, dropped into the fan, zapped by spark plugs, connected to 240V mains, and we can't tolerate a single battery combustion incident.

Please feel free to ask questions, make suggestions, or solve the problem!

thanks, Roger

Reply to
rkl
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On Apr 16, 9:37 pm, snipped-for-privacy@veecad.com wrote: [...]

I assume that the device has a micro in it etc. that remembers things and uses the battery as a power source to hold that memory up.

I don't see any problem with having the micro also remembering the time of the last charging of the battery and other stuff like that. You can gate the charger onto the fast charge mode only if the estimated state of the battery is nearly discharged.

You can charge at a very low rate almost all of the time. I don't think having two levels of charging makes the circuit much more complex.

I'm thinking of a circuit like this:

Two bits from the micro run to level shifters that make 0 to +12V gate drive signals of them.

Two N - MOSFETs with source resistors that will bias on a couple NPNs if the current tries to go too high. Note: I assuming the positive end is connected directly and the neg. is the one switched. Turn stuff around if you can't do this.

The NPNs pull down the gate drive of the MOSFETs to regulate current.

When the external battery is disconnected, the body diodes of the MOSFETs conduct allowing the battery to take over.

An ADC channel in the micro circuit will monitor the battery voltage so the micro can decide.

Reply to
MooseFET

[...]

What's the "have to" business?. Is it specific to your industry, or are we all going to be buggered by new regulations?. Personally I wouldn't choose to specify anything other than sealed lead acid. It's Victorian technology but a damned sight more reliable/predictable/cheaper than the newer fluff.

--
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Reply to
john jardine

I would not say this is always the case. Qutie a lot of work regarding NiCd behavior was done for space missions, for example, and those applications did not generally involve full cycles or fast charges.

I am aware of three types of charge that NiCd and NiMH can commonly accept:

  • Fast charge, 1C or more, with appropriate dV/dt or dT/dt termination
  • C/10 charge, for several hours after it's full but not forever
  • C/300 charge forever

You can get a fairly good impression of the state of these cells from the voltage, perhaps temperature compensated - certainly better than you can get from a lead acid using just the voltage. It seems like the best approach for you is to set a charge cutoff below which you will perform a fast charge and above which you will perform a slow charge.

I would recommend NiCd, which is more robust in most ways than NiMH, and is probably the most robust and least expensive technology remaining once you eliminate lead acid for whatever reason is forcing you to do that.

If you can accept less than a full charge, you can charge to a (temperature-compensated) voltage level; you can't get to 100% using this technique like you can with lead-acid, though, because the voltage starts going back down for reasons my chemistry is too weak to properly understand.

Forget about lithium; the safety issues will double your engineering work in exchange in an application where none of the advantages matter much. If you have to go with them, look at A123's cells.

Reply to
Terran Melconian

NiMH are more like NiCd than before, so much of your NiCd ideas now apply to NiMH. GP Batteries NiHM specs permit C/10 for 12 months, C/

20 forever. The NiCd space application angle is a good idea - I look up info.

Lead and Cadmium are out, this is for EU.

Thanks. I think something similar could work with NiMH. Switches to C/10 when voltage comes up above a set point, below that current increases to C/3 etc. Temperature compensated.

Your advice much appreciated.

Reply to
rkl

RoHS lead and cadmium are out. We had a good run with SLA.

Reply to
rkl

Time for re-design that does not require any nternal battery. Run your circuit from the battery under test. Charge up a supercap from that battery if it needs memory between events. You'll probably need to re-design for low current. If an automotive battery can't provide enough power to drive your circuit, it's shot anyway - no need to test further.

Ed

Reply to
ehsjr

I like it! Actually, many low end testers do this. Once you add stored results, printing, backlight, test from flat etc you do need a battery.

Thank you all for creative and useful input.

I'll follow up microprocessor with coulomb counting, or voltage controlled NiMH charging. LiPoly is still a posiblility, because of its excellent voltage vs state of charge characteristics. Plus I will check with management to see if we can hang on to lead-acid a while longer.

Roger Lascelles

Reply to
rkl

I saw an article/advertisement in an electronics magazine (electronicsweelky.com) about float charging Lithium cells:

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BTW, the failure mode I have seen in all the laptop batteries that I have opened is open-circuit cells, so for reliable operation I would consider multiple lithium cells in parallel (with whatever fuses etc. are needed to make this safe).

Why are you designing out the SLA?

Chris

Reply to
Chris Jones

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