12V 90Ah Lithium Ion Battery Construction - Newbie

NDStamsta

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Feb 21, 2018
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Hello All,

I apologize in advance as I am relatively naive to the world of lithium ion batteries. I am looking to construct a 12V 90Ah battery from 26650 3.7V 5200mAh cells. First, how should I configure these cells for spot welding the nickel plating? Series, parallel, yeah yeah... diagram please!

The load this battery will sustain will never be more than 30Ah. Knowing this, what battery management system should I select?

Last, what is the quickest charger I can select based on the 12V 90Ah battery with what I imagine is maybe a 35Ah limiting BMS?

***I realize exactly a 12V 90Ah battery isn't possible, my guess is with the cells configured properly I will arrive at an 11.1V 93.6Ah battery pack.

Thank you all for your help, it is greatly appreciated.
 
NDStamsta said:
First, how should I configure these cells for spot welding the nickel plating? Series, parallel, yeah yeah... diagram please!
YOu'll want ot read up on all the various 18650 / 26650 "how to" and "build" threads, to get an idea of what NOT to do, and then choose one of the many ways to do it.

The load this battery will sustain will never be more than 30Ah.
If you'll never use more than 30Ah of capacity, then why build a 90Ah pack, which will be 3x the size and weight you actually need?


Last, what is the quickest charger I can select based on the 12V 90Ah battery with what I imagine is maybe a 35Ah limiting BMS?
BMS don't limit your capacity, so there arent' any that will limit to 35Ah. Aside from the issue that there's no reason to build a 90Ah pack that you'll only use 30Ah from (or 35Ah, you seem uncertain?) you may be confusing Amps (A) with Amp-Hours (Ah). They're completely different things, so you need to be sure to use the right term in the right place. If you're not sure what they mean, you should look at the ES Wiki linked at the top of every forum section in the stickies; it has articles about that.
 
If you'll never use more than 30Ah of capacity, then why build a 90Ah pack, which will be 3x the size and weight you actually need?

If the battery is 12V 90Ah, then it will be able to support a 30A draw for 1.5 hours, correct? Or am I completely off base?
 
NDStamsta said:
If the battery is 12V 90Ah, then it will be able to support a 30A draw for 1.5 hours, correct? Or am I completely off base?

Correct. It could go for 3 hours in theory, but the battery will live longer if you use less of the capacity on each cycle.

The main problem is no combination of Li-ion cells will give you 12v. 3 series will be a bit low and 4 series will be a bit high. Depending on what your load is, you might choose one or the other. Another option is to go with 4s and use a dc-dc converter to get a regulated 12v. This is less efficient and costs more, so should be avoided if possible.

Most devices made for automobile systems can handle 15v.
 
fechter said:
Correct. It could go for 3 hours in theory, but the battery will live longer if you use less of the capacity on each cycle.

The main problem is no combination of Li-ion cells will give you 12v. 3 series will be a bit low and 4 series will be a bit high.

Thank you for the reply. The load will be 12 60W peltier plates (12V 5A). I realize 3S Li-ion will only reach 11.1V, will this be a major deficit to the load performance?
 
Peltier devices can operate over a wide voltage range. The heat flow will be roughly proportional to the voltage.

3 series would range from about 12.6v hot off the charger, down to about 9.9v at fully discharged. Average during discharge will be around 10.8v.

4 series would range from about 16.8v to 13.2v, so quite a bit higher. 3 series seems to be closer to what you want.
 
3s18p will get you about 11.1 volt 93ah with those 5200ah cells, 3s17p wll get you 88ah.

You can use any 3s li-ion bms that can handle 30 amp discharge, some are 5 dollars, some cost 80 dollars, they all work about the same except the more expensive ones can handle higher charge rates, the cheap ones max out at about 10 amps charging. Just make sure the bms is for 3.7 volt cells, some bms are for the 3.2 volt lifepo4.

You can easily charge it at 30 amps or higher as long as the nickle strip/wiring/connectors can handle it. Balance charger is the safest way to charge these batteries, I use isdt q6 14 amp charger, they make one that goes to 30 amps. On all my li-ion packs I been using fuses on every cell. I would definitely recommend it.

11.1 volts is very usable in 12 volt devices, I never had any of my inverters shutoff from low voltage.

things needed
-10pcs JST-XH 3S Lipo Balance Wire Extension Charged Cable Lead 11.8" 30cm (needed to check balance and connect to bms)
-xt60 connectors (these are for all the high amp wires, also to connect to most chargers)
-3in1 LCD Li-Po Battery Balancer+Voltage Meter (to view pack balance status)
-LCD 12V Battery Capacity Tester Indicator Car Lead-acid Lithium Voltage Meter (to check pack voltage/percent available status)
-28 gauge solid copper wire (not strand) this if you are going to put fuses on your cells. Use for fuse wire.
-12 volt overvoltage relay (these costs about 6 dollars) highly recommend you get one these, you can use these to shutoff charging when the battery reaches a certain voltage, in case bms fails this will save battery pack. With one of these relays you can charge with any 12 volt charger, it monitors pack voltage and disconnects charger when it reaches set voltage.

This a picture of my 3s30p 65 ah 11.1 volt battery pack, I use 18650 cells and solder everything. This is a bad design not enough series connections but it rarely sees more than 12 amps discharge. On my next battery pack I will use a tab welder and 28 gauge copper fuse wire instead of the pin fuses I used on this pack. You learn with every pack you build, and the users in this forum posted pictures of some of there designs.
a 90 cell lithium.jpg

inside its case
a 90 cell case.jpg
 
NDStamsta said:
If you'll never use more than 30Ah of capacity, then why build a 90Ah pack, which will be 3x the size and weight you actually need?

If the battery is 12V 90Ah, then it will be able to support a 30A draw for 1.5 hours, correct? Or am I completely off base?
It depends on the cells used, and the rate they can output current at, but 30A would be 0.333C for a 90Ah pack, so it's likely the cells could handle it easier that way, which will make them last longer.

As long as the cells have at least a discharge C-rate of at least 0.333C then they'll work for this. The higher the cells' C-rate is, the better off they'll be.

A 90Ah pack, fully charged, could support a 30A draw for 3 hours, if you ran it all teh way down to completely empty (but that's harder on teh pack).


Keep in mind a 30A current draw has nothing directly to do with 30Ah capacity. (A does not equal Ah; they're not interchangeable terms)
 
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