Replacing lead acid to 18650 on my garage door opener.

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Auzunder

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Sep 14, 2021
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I have a motorized garage door, and in the last years I'm having issues with the 12v lead acid battery that came with the kit. The battery outputs only 0,5A in short circuit and the voltage drops to less than 8V when the door tries to open. It opens 1 time and them the battery is dead, not having a chance to close the door.
Ofcourse the battery is only used when the grid is out of power, but when I need it's anoying to have the door open until the grid has power.

I already have a bunch of 18650 from my last salvage laptop batteries and I wanted to reuse them. My problem is charging the batteries. I saw that the open circuit voltage of the door opener charger is about 13.8v. And for 18650 3s pack is too much and for 4s is too little.

So, I thought on having a seperate 12,2/12,4v charger connected to the batteries and soldering an diode in one of the terminals of the battery in the garage door opener...

My question is: Is this enough to charge and when I need the batteries they are charged with no problems? Or the 2 chargers would conflict being one 13,8 having a diode stopping from charging the battery and another one at 12,2v charging the battery?
 

Korishan

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My suggestion, either go with LiFePO4 cells, or stay with lead acid. For something like that, you aren't really sacrificing or gaining much.

If you really want to go with 18650's, then you need a proper charger that you can set the charge voltages for. You can't use any single cell chargers and put them in parallel without adding a lot of over head circuitry.

The problem with using a standard charger is that it won't charge lithium batteries with the proper charge curve. You can't use a lead acid, or dumb charger, to charge lithium cells/batteries and expect them to last long.

The diode probably won't work properly as the charger won't see the voltage on the other side. Or, it'll continue to try to dump top voltage in regardless. So you'll be wasting power in the end, and potentially overcharging the lithium battery.
 

italianuser

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Feb 25, 2020
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Hi there. As you noticed 12V batteries are more difficult to swap out with 18650 ones than 24V ones.

Normally you would use a 4S (14.8V) configuration and not a 3S which has a too low voltage (11.1V).

What Korishan says is correct.

If I wanted to try using 18650 I'd start with something like this:

- A 4S charger with an output of about 1/2 the amperage of the lithium battery. For e.g. if battery is 10A the charger could be 5A, that's safe/good enough for most lithium batteries.
- You should use high drain 18650 cells, for e.g. INR chemistry ones which can give 20/30A output each. You can find this out by checking the cell's datasheet here in the forum. If you want to use "normal" ICR cells you must be sure that when the motor is on it wont ask for mure current than what the cell can give (check the watts the motor needs).
- A DC-DC step down to put in between the battery and the load. You want to lower the 14.8V nominal voltage of the lithium battery down to 12.8V. Not sure if there's anything better than a DC-DC stepdown but I'd start checking this out. It must be capable of giving enough current in output for your motor. A 500W motor at 12V is a 41.6A load.

Keep us updated!
 

Oberfail

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Jun 22, 2021
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just use a buck converter to change the charging voltage
i'd go for 4s charging the batteries up to 15v (3.75v), that should still be within the limits of the door opener circuit and keep the batteries at a happy charging state, so they dont degrade as fast.
 

Korishan

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just use a buck converter to change the charging voltage
This is still ignoring the charging curve that is needed for lithium batteries. You can't just dump full current into them until they reach top voltage. It'll shorten the life of the cells and they'll need replacing again. Usually at the worst possible time.

The other issue I didn't mention is the "isolation" of the battery. It's up in the air, in a sealed box, that can't be accessed quickly. If something were to go wrong with the battery, there's no easy way to get to it.
I suppose if you mounted the battery "away" from the motor housing in a sealed metal box, you may be able to make this safer. But the wires still have to come out of the box.
 

Oberfail

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This is still ignoring the charging curve that is needed for lithium batteries. You can't just dump full current into them until they reach top voltage. It'll shorten the life of the cells and they'll need replacing again. Usually at the worst possible time.
Lead batteries should be charged at 0.2c, most door openers have a battery with 5-10Ah, so the charger itself should already be limited to 1-2A.
And i also doubt, that he is going with a 4s1p config and even if, the charging current should still be fine, discharge however might not.

While yes, its not a ideal way to charge, it still works just fine, especially with a bms to keep the batteries within their limits. Which i especially suggest here to use, since the battery is probably going to be put into the opener on the roof, so its not easily accessible once its tucked away.
 

Korishan

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I'm not talking about the charging "current", I'm referring to the charging "curve". aka, CC/CV. If you charge lithium at a steady current up to voltage termination, you never really fully charge the cell/battery. The charge current needs to taper off the last 10% of charge to get the capacity back to full. Even if they aren't charged to full 4.2/cell.
Also, most lead acid chargers "float" charge. This is bad for lithium. That's actually worse than not following the charging curve. Trickle charging lithium will shorten its life
 
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