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I have a 36 V lawn mower with a battery container with three 12 V SLA batteries in series.

I am thinking about switching to LiIon batteries for more run time (and hopefully more recharge cycles). I can find batteries that fit physically, and putting them in series is easy.

I don't think the SLA charger will charge LiIon batteries correctly. I found 36 V LiIon charges online. But...it is safe?

  1. I thought LiIon batteries packs need a thermal sensor input to the charger to prevent overheat/damage during charging. All the LiIon charges I found are 2 pin output.
  2. Is it safe to charge 3 LiIon in series? Or do I need to charge each 12 V individually?

I found someone doing exactly this project but using small cells (YouTube). However, he does not discuss if/how he changes charging circuit or running circuit.

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closed as off-topic by Chris Stratton, RoyC, Dwayne Reid, laptop2d, Dmitry Grigoryev Nov 12 '18 at 8:09

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Chris Stratton, RoyC, Dwayne Reid, laptop2d, Dmitry Grigoryev
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ This is not really a practical project. Different cell chemistries have drastically different behavior under charge and discharge, so it is not only the charging circuit which will have to be entirely replaced, but quite likely the run-mode electronics as well. The details of doing this properly are far beyond what would fit in a post here, and require information about the system which you do not seem to have. Because this is asked from an application perspective rather than a design one, it will have to be closed as an off-topic "usage" question. This project is unsafe, do not attempt. \$\endgroup\$ – Chris Stratton Nov 6 '18 at 14:41
  • \$\begingroup\$ 1. LiIon cells need special chargers which are most of the time specific for the battery pack. Temperature sensors can be one option for safe battery charging. 2. See 1 3. The electronic in the mower is calculated for a special voltage and current range. It is possible, that the motor is drawing too much current over time or brings the cells into deep discharge condition if not monitored correctly. \$\endgroup\$ – A.R.C. Nov 6 '18 at 15:30
  • \$\begingroup\$ Each 12V Li battery will have 3 cells in series (and possibly several parallel legs, and typically include a charge controller which prevents over - and undercharging, and maintains cell balance. If you connect three batteries in series, you don't have any balancing between the state of charge of all the cells in series, and over time the SOC will drift off. The undercharge protection is a last ditch protection from allowing the chemistry to change - which is dangerous, since the cell starts to produce hydrogen. It's not worth the risk of doing this without a proper controller for the set. \$\endgroup\$ – Phil G Nov 6 '18 at 16:07
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I wouldn't charge separate packs in series.

If you have a pack with series cells (if it is a 12V one, it must have series cells), the balancing electronics is built into it if it is any good. (As well as the emergency shutdown mechanism.) If it doesn't have it built into it, the battery will blow up in your face during usage anyhow, so why worry about charging :D

If you have separate cells you can charge them with, per se, a lab power supply, or with any other power supply on which you can set a maximum current and a maximum voltage. They won't explode. The problem arises when there are multiple cells in series and one of them charges faster then the rest of them. That is the one which will blow up.

(But to be honest, in my experience, if the cells are from the same batch, the aren't likely to fall out of step with each other.)

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  • \$\begingroup\$ I see LiIon chargers all over amazon for 36 V battery packs, that say 42 V output, and all have 2 pin connections. I looked at 12v LiIon batteries and none explicitly say they contain electronics to limit charging based on heat/current/voltage. So that makes me nervous. Wonder if I'm setting myself up for disaster. \$\endgroup\$ – TSG Nov 7 '18 at 0:20
  • \$\begingroup\$ would be helpful if linked the links... Also, why don't you by a 36V battery to start with? \$\endgroup\$ – Cerike Nov 7 '18 at 0:34
  • \$\begingroup\$ Good point! I was trying to replace the 3 SLA batteries with 3 LiIon batteries, but I will investigate something premade that fits into the same case. (Perhaps dremel out the plastic guides in the case). \$\endgroup\$ – TSG Nov 7 '18 at 13:49
  • \$\begingroup\$ I have split my question - leaving this one about charging. Once I figure that part out I will ask about operations. \$\endgroup\$ – TSG Nov 7 '18 at 14:09

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