Repurposing 13s4p and 14s4p 18650 packs for 12v duty

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killerniceguy   1 µW

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Repurposing 13s4p and 14s4p 18650 packs for 12v duty

Post by killerniceguy » Feb 18 2018 11:17am

I did search but perhaps I am not asking the right question...I am also pretty new to this stuff and eager to learn, clearly there are some gaps in my knowledge.

I have two Ebike batteries (13s4p and 14s4p 18650 with BMS) I would like to use to run a 12v powered cooler while I am traveling with the family this summer. I could easily pick up a sealed 12V AGM battery but I would like to see if this is possible and or a good idea (maybe this is a bad idea...).

The cooler I plan to get is:
https://www.dometic.com/en-us/us/produc ... w-_-152179

Cleary I will have to step down the voltage to 12V. Something like this:
http://www.current-logic.com/shop/index ... cts_id=259
will step it down to 12V and also meet my amperage requirements of roughly 5amps at startup. The cooler typically pulls >1Ah during operation.

After doing some reading my concern is the power coming from the converter will be modified sine wave and not very good for the compressor in the cooler. Is there a way around this? Is there way to make it a pure sine wave?

Other info and assumptions that may be important:
I have a 72V Cycle Satiator I use to charge the batteries
I do not want to rebuild the packs in anyway - I will use them as ebike batteries when I return from this trip
I am not planning to charge these packs with solar although if there was an easy way without rebuilding the packs I would.
I figure I get about 10Ah from each pack, if I convert to 12v I should see about 40Ah per battery less the inefficiency of conversion right?
The cooler has a low voltage cut off of about 10.7V, will this factor into available power reserves from the 18650 packs as the cutoff was put in place for Pb batteries?

I appreciate any input on this project

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amberwolf   100 GW

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Re: Repurposing 13s4p and 14s4p 18650 packs for 12v duty

Post by amberwolf » Feb 19 2018 3:00am

killerniceguy wrote:
Feb 18 2018 11:17am
Cleary I will have to step down the voltage to 12V. Something like this:
http://www.current-logic.com/shop/index ... cts_id=259
will step it down to 12V and also meet my amperage requirements of roughly 5amps at startup. The cooler typically pulls >1Ah during operation.

After doing some reading my concern is the power coming from the converter will be modified sine wave and not very good for the compressor in the cooler. Is there a way around this? Is there way to make it a pure sine wave?
I don't think I understand the problem, or maybe which equipment you're going to use.

IN general, if you have a 12v unit, it's almost certainly DC, not AC powered, and so you can't use anything with an AC (sinewave or otherwise) output at all.

Things with an AC output generaly are outputting 120VAC or thereabouts, which would also be about 10 times the voltage your device can handle on it's input. and would probably destroy it.


If instead you're going to convert the ebike battery power to 120VAC first, then power the device from it's own 120VAC-to-12VDC "wallwart" power supply, then keep in mind you'll be having a doulbe efficiency hit. So if it's say 75% efficient to convert your ebike power to AC, and 75% to convert that AC to 12VDC, then it's 75% x 75% = only 56% efficient overall.

As for whetehr the device's wallwart runs on modified sinewave, you'd have to check with the manufacturer; some do and some don't.


That said:

If I read the cooler page correctly, it says it runs on 12vdc, or 24vdc, or 120VAC. So you can use a step-down DC-DC with an input range at least as wide as your ebike batteries full-to-empty voltage, (each one has a different range) and an output voltage of either 12vDC or 24VDC.

Theoretically, the 24VDC should require less current, and be more efficient (less of a conversion down from the biek batteries), but that depends on how they designed the cooler.

Similarly, you can use a 120VAC output converter (again with an input range at least as wide as your ebike batteries' voltage range), but you'll probably want to check with the manufacturer about the sine vs modified sine issue, as well as startup currents and type of load (some converters don't handle inductive loads well).

And again, it depnds on teh cooler design if this is more or less efficient than running it on DC.

I figure I get about 10Ah from each pack, if I convert to 12v I should see about 40Ah per battery less the inefficiency of conversion right?
Depending on the converter and the specific load, you might get anywhere from 50% to 85% efficiency. You'd ahve to either test it when you get ot to see what it's efficiency is, or check with the manufacturer's spec sheets (if there are any).

THen multiply that effiency times the maximum capacity your packs can output *at the rate this load will use them at*.


The cooler has a low voltage cut off of about 10.7V, will this factor into available power reserves from the 18650 packs as the cutoff was put in place for Pb batteries?
Not directly--only in what your DC-DC converter has to be able to sustain output.

It *would* matter if you were going to directly power off the cells int eh packs, rebuilt into lower-voltage packs, but you're not doing that.

killerniceguy   1 µW

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Re: Repurposing 13s4p and 14s4p 18650 packs for 12v duty

Post by killerniceguy » Feb 23 2018 8:24pm

Thanks for the reply and for the information!

I have a question regarding switches:

My battery pack is terminated with a XT90 anti-spark. After the XT90 I would like to put a switch between the battery and the fuse. After the fuse will be the 48V-12V converter, I am planning to use an ATC 20 amp fuse, my BMS also has a 30 amp cut off. I am not sure what switch to use, almost everything I can find is 12V-24V and anything 48V is really big (physically) and takes up a lot of real-estate in the ammo box I am building this in. What is the danger of using a 12v/24v 30 amp switch for something like this?

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Re: Repurposing 13s4p and 14s4p 18650 packs for 12v duty

Post by amberwolf » Feb 24 2018 1:40am

If the switch is not switching under load, then a lesser switch can be used.

If the switch must be able to disconnect or connect with a load (current flowing thru the switch), then the switch has to be rated to disconnect that voltage (or higher) at that current (or higher) to be guaranteed to perform the disconnect (and/or to not damage/destroy the contacts during connect).

If a switch is only rated at say, 12vdc, and you use it to disconnect a "48v" battery system (that is probably closer to 60v fully charged) that's five times teh voltage it's rated for. So it's that much more likely to continue arcing across the contacts on disconnect under load then it would be on the 12v system it was meant for. To prevent that, the switch is bigger to have more space inside it for more contact distance when open to break the arc, and sometimes for magnets to help do that.

If a siwtch is rated for AC and has no DC rating, then it may be unable to break the arc of any DC voltage, and just weld itself on when engaged, and unable to switch off at all. The higher the DC voltage and current it's used with, the more likely that is to happen.


Whether any problems will actually occur depends on what's happening at that moment when the switch is opened or closed--hardly ever are there problems when the switch is already closed and staying that way. :)




FWIW, I would have the fuse as the very FIRST thing right after the battery output terminal. That way if anything ever goes wrong, the fuse will blow and disconnect the battery from the entire wiring harness past the fuse.

The more stuff that's between the fuse and the battery, the more chances there are for things to still connect across the battery terminals and cause fires (because the fuse can't protect against stuff it's not connected to).

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