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Battery for Portable Freezer

It is time for me to stop lurking and try my hand at recycled 18650's.

Looking to build a battery to power our portable freezer at the Farmers Market. It is currently being powered by a 12 volt deep cycle lead/acid that is hooked up to a solar panel to keep the battery charged.

The freezer is able to run also at 24 volts so was thinking to build a 8S 20p battery which if my math is correct would give around 44 amp hours, and be about 26 volts. The freezer pulls 2.5 amps at 24 volts so that should give me around 17 hours of run time. Is that correct or am I looking at it incorrectly?

Any suggestions are welcomed as nothing is in stone yet, and this will my 1st project dealing with 18650’s.

Thank you for your suggestion

You should go 7s if you plan for LiIon?

That is a range between 21 to 29.2V. 8s goes all the way to 33.6 (If you utilize full range)

You are basically correct in your calculations.

Just build it. Either you get a solar panel with close to same voltage as the battery and use a BMS with charge and discharge port separated. (Its uggly hack to charge but works) or even better get a MPPT charger in smaller format and hook up.

The later with MPPT charger is 10x more recommended. Dont forget the BMS to protect from over and under voltage on the setup.
Happy build!

This from my chan could potentially give you some ideas.
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Thanks Daromer for the reply. I had thought about 7s and in a lot of ways make more sense so I am going to change to that. 

Any suggestions for a Small form MPPT? 

Also since the output range of 7S is between 21 to 29.2v will I need to include a converter to maintain 24 volts?

Thanks again for the suggestions.
So onto my next question which is around BMS.

So I have already built one 7s20p battery and have added a BMS to it and the battery is working great. But now I am thinking about building a 2nd 7s20p battery and connecting the two in parallel. From my understanding I will not want to add a BMS to the new battery as it if I did the two BMS's would fight with each other. But instead would want to parrallel the packs to one BMS. So basically leave the BMS connected as is on the first battery and then connect the corresponding battery groups on the 2nd battery to the 1st with jumper wires.Hopefully that makes sense.

Is that correct or would I want to add a BMS to the 2nd battery? Or would I need to wire it differently that I am not thinking of? But I do not want to pull the build battery apart if at all possible.
If you parallel each pack in the 2 strings, then yes you need 1 bms. If you only parallel the 2 leads coming from the 2 strings, then each string needs its own bms. The latter would connect each 'after' the bms
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That's what I do with my packs. I currently have 4 7s60p packs linked together. The two ends of each string are combined with 4 gauge wire to the inverter. Then for the in between cells, they are linked together via small 12 gauge wire as it is only balancing current in between cells.
Large-format LiFePO4 cells can be a big investment, but they are often used in 4S for a 12V lead-acid replacement. 

As others have said above, if using 18650-format cells in the NCA/NCM chemistries (those are the common Sanyo, Sony, Samsung, Panasonic, LG cells), then 7S is well-regarded for 24V system. I have seen 13S and 14S used for "48V" systems, and I am using 14S electric bike battery packs for a back-up inverter.

LiFePO4 is a nominal 3.2V
NCA / NCM is the common 3.7V

Be aware that many of the builders here have invested in the infrastructure to assemble and swap-out 18650 cells because sometimes we can find a source of "near free" cells that still have 80% of their life left in them. Laptop cells are low amp, and are designed for lowest price. Cordless tool cells are the same format, but they are very expensive because they try to pack long range with high amps.

Right now I would guess that the medical pack cells occupy a middle ground and are the best bang for your buck. By that I mean...if you have access to lots of free laptop cells, you may need a very large pack to provide the amps needed to handle the start-up current on the freezer compressor (1A-2A per cell). Brand new cordless tool cells would be $4-$6 each, so...very expensive, but they would accomplish your goals with a fairly small pack. I think you might be able to get medical pack cells for about $1-$2 each, maybe 5A each?

I've seen home packs with 80 cells in parallel, so 7S 80P would be 560 cells, and 12V X 160A (2A per cell) would be 1900W

So many variables...
(05-09-2019, 01:45 PM)HikingGuy42 Wrote: Any suggestions for a Small form MPPT? 

I use MPT 7210a -  $47'ish to charge my home-built 7s60p / 130ah@24v / 3,300watt portable battery.   Its in large cooler with wheels.  

Google / youtube and you'll find good info/videos.   Its pretty cool because it can accept a wide variety of DC input but deliver a fixed output for charging and will even boost the input UP to output needed.   I use a pair of Renology 100watt panels for solar charging AND a 120v powered 24v power supply as inputs so I can do Solar or 120v charging.

A bit more expensive is this inverter + built-in PWM Charge controller.   I haven't used it but it was features in this youtube
I have had good luck with the Genasun boost mode controllers.  It will boost 12-18V from a solar panel to the appropriate charging voltage.

I do not recommend the MPT7210A.  I've had nothing but trouble with that thing.
In terms of quality boost controllers, Genasun makes a great controlled if you can afford to spend the money. I use their regular buck controllers for years, but since the Victron stuff has come out with built in bluetooth, I have switched to those for my bucking needs.

I still have 2 Genasun controllers and no plans to get rid of them.

Many of the boat, RV, and marine people use them as well because they are often times used on a 1:1 panel to controller to combat against shading. That said, Victron handles high voltage (panels in series) much better.

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