How Much Is Too Much?

Nuggets69

Nuggets69

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I finally found a decent supply of batteries andhave been processing my 18650'sfor a few weeks now. Ihave been checking the internal resistance on each battery after it has been fully charged to 4+ volts. So how much internal resistance is too much? I have tested some that seem excellent at 75m ohms and 2900 mah, and others that seem bad at 300m ohms and only a 700 mah rating. I still there a specific point where the IR istoo high and the capacity is too lowto be considereda good cell or is it gauged by howwell the cell charges and discharges?
 
If you can test them at the current that you need max then the Ir is low enough!

With that said generally below 100 is like new and below 200 is not a problem and below 250 still works.

Dont forget that some testers are crap in terms of testing like the opus. The opus first of all adds up 10-50mOhm and then you have contact issues. Take a look at my 2nd half:

I dont test the Ir on my cells. I only test so that they can do 1A without issues. Since my max currently is like 300mA its more than enough.
 
I am attempting to build a 48v battery bank for off grid homestead applications. I just wanted to try and match the cells as close as possible so I don't have issues with mismatched cells pulling down the rest of the cells. Or does that only apply with the battery capacities? I have a huge capacitance range between the cells I have harvested. They range from 1500 mah to 2900 mah. Should I group them for a more reliable battery bank or does it even matter with this many individual cells?
 
What you want to do is build parallel packs with the closest capacity variation as possible, and this is the same in series
 
Parallel packs should be close from cell to cell. I wouldn't go any further than say about 300mAh variation per pack. You could go to 500mAh, but that'd be like the absolute max recommended.
For Serial packs, it's a bit more picky about variations. So from pack to pack you probably wouldn't want any variation higher than 100mAh. The closer to 0mAh variation, the better, especially with serialized packs/cells. This is to keep the packs in the serial string balanced with each other voltage wise; in turn, this keeps from having to balance the packs by burning off the excess energy stored on the higher voltage packs.
 
Serial strings doesnt matter if they are close or not when we talk about LiIon cells. They will balance out in total. I got strings with 200Ah and I got strings with 35Ah and they will charge full at same time and get empty at same time.

The thing you should be close at is the packs in a string. Its not a must actually but you cant use more than the pack that have the least. If you have 3 packs lets say 20, 20 and 10Ah the max you can use is 10Ah. This is though not really good in practical point of view but still.

I wouldnt say go within xx mAh. Its more important % wise. I have alot more than 300-500mAh difference in my big packs. More like 10Ah difference but they are also HUGE in terms. Not in purpose but thats how it is :)

I though mix my packs closer to 100mAh but thats random mixed so worst case is that i have like 8AH out of balance on my 80p packs. Thats up to 5% or something.
 
Thanks for all of the great advice guys! I have thought about buying a IR meter, but they are a bit pricey. The high IR cells heating up should be a red flag for sure. I assume you are charging at 1000 mah to make sure the bad cells heat up?
 
We charge at 1A. (1000mA)
Note that Ah is capacity that you measure whats its capable of storing.

You can measure internal resistance pretty good with many RC chargers but many of them are also way off.
Here is a video i tested one
 
What cell configuration would you recommend for a 48v battery for my RV? 14s12p ?
 
14s for 48V. Xp for how much amps your going to pull max. This number is based on what mA the cells were tested at. If they were at 1A and you need 40A, then 40p. If they were at 500mA and you need 40A, then 80p
 
The electrical load, the breaker, or the alternator output?
 
50A for an RV usually means the max AC amperage coming into the RV into the fuse box. Usually RV's are 12VAC so your basic 4s setup and with as much capacity as you can fit. It's common to find 12V 100AH / 200AH LION batteries but they're crazy expensive.
 
Ok, so if the breaker box is rated at 50A, that doesn't mean running all the equipment in the RV will come even close to that. If the max amps of consumption is 20A, then he doesn't need to build a pack that can do 50A. This allows for smaller packs to make them fit easier in the allotted space.
That's why I asked if the 50A was the load, or the breaker. It makes a huge difference on the requirements of the pack size.
 
If the RV is rated at 50A max, it means it can support 2 AC units, or 1 AC unit and a pre-wired spot for a 2nd AC. Typically during startup is when you need up to 50A when both compressors kick in but once the initial surge is over, it likely doesn't need more than 20A as you said. Some people are implementing a device that prevents the initial high draw from the AC units to allow 2 AC units to run off 30A circuit. Can't recall what it's called.
 
Yes, it does have two AC units. One is a 13500 but and the other is a 10500 unit. I can run one AC on my 5500 watt generator with a 120v 20 amp circuit, however I burned up the generator section of my generator trying to run both AC's on the 30 amp twist lock RV circuit. The surge was just too much for it. I would be happy if I could run just one of them off the batteries. But that presents a whole new set of issues involving enough amp hours of batteries and sufficient solar panels to keep it all charged. I am not an engineer so figuring it all out has been a bit more tasking and difficult than I anticipated.
 
Ok, So if I am doing the math correctly. For a 48v battery bank that is over 50 amp hours and assuming that each Li battery averages 1.5 amp hours. I would need a 14s35 battery. Can anyone confirm the numbers for me?
 
Nuggets69 said:
Ok, So if I am doing the math correctly. For a 48v battery bank that is over 50 amp hours and assuming that each Li battery averages 1.5 amp hours. I would need a 14s35 battery. Can anyone confirm the numbers for me?
Click to expand...

That's about right. I get 14s34s for a nice even parallel number of 34.(34 * 1.5Ah = 51Ah).

Mind you, you can't use 100% of that 50Ah so consider adding 20-30 more Ah which is 70/80Ah to have 50Ah usable.

EDIT: Something like this;
 
Nuggets69 said:
Ok, So if I am doing the math correctly. For a 48v battery bank that is over 50 amp hours and assuming that each Li battery averages 1.5 amp hours. I would need a 14s35 battery. Can anyone confirm the numbers for me?
Click to expand...

If you're running on lead acid batteries right now, what are the amp hour ratings on them and voltage per battery? Then you can calculate an equivalent li-ion pack size that will replace your current batteries. If not, you might want to get a kilowatt hour meter or something that can calculate your actual power usage.

Otherwise trying to get 50A is just for circuit max does not equate to actual power usage. It just calculates the minimum pack size that you can safely draw 50A. Your A/C might surge but it will run at much lower amps. Unless you plan to use the batteries at exactly 50A for one hour which is what you're specc'ing at. Also you can look at soft start capacitors that you can lower the surge at the compressors.
 
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