How to read AH in my batteries?

[oshky]

Hello,

Recently I added two batteries to my bank for a total of four. They are in parallel and sum 100 Ah x 4 = 400 Ah. How can I read the batteries to know if actually I have the 400 Ah I'm looking for. I have a voltmeter, would that help?

Thank you!

 

[OffGridInTheCity]

You need to do a discharge test. This process involves the following steps..
1) Do a full charge of the battery.
2) Place a known load (known/fixed/steady watts) or use a tool such as an iCharger X8 or a killowatt meter in conjunction with a load.
3) Discharge down to the cut-off voltage - presumably you have a BMS with cut-off, so discharge till the BMS cuts-out.
4) Read the results and figure what you 'actually' have.

For example, let's say you have a 12v -> 120v inverter, you could use a killowatt meter (120v load measurement unit that's not too expensive) connected to a hot plate of heater or some other load around 700w or so.
Note: Its common to pick a discharge standard such as 0.2C which you can determine by the specs of your battery. Its not that important for a ball-park test but the idea is to do a solid but ordinary load so it doesn't take a week but won't do max current pull from the battery.

Let it run till BMS cut-off (hopefully you have a BMS) and the killowatt meter will show you the total watt-hours. The inverter would have 10-15% loss to add in - so could add that in to the total.

A 400ah battery @ 12v should have around 4,800watt-hours at 100% discharge. In theory, a 700w load (4800wh/700w per hour) will run for 6.86hrs.

 

[Short_Shot]

You need to do a discharge test. This process involves the following steps..
1) Do a full charge of the battery.
2) Place a known load (know/fixed/steady watts) or used a tool such as an iCharger X8 or a killowatt meter in conjunction with a load.
3) Discharge down to the cut-off voltage - presumably you have a BMS with cut-off, so discharge till the BMS cuts-out.
4) Read the results and figure what you 'actually' have.

For example, let's say you have a 12v -> 120v inverter, you could use a killowatt meter (120v load measurement unit that's not too expensive) connected to a hot plate of heater or some other load around 700w or so.
Note: Its common to pick a discharge standard such as 0.2C which you can determine by the specs of your battery. Its not that important for a ball-park test but the idea is to do a solid but ordinary load so it doesn't take a week but won't do max current pull from the battery.

Let it run till BMS cut-off (hopefully you have a BMS) and the killowatt meter will show you the total watt-hours. The inverter would have 10-15% loss to add in - so could add that in to the total.

A 400ah battery @ 12v should have around 4,800watt-hours at 100% discharge. In theory, a 700w load (4800wh/700w per hour) will run for 6.86hrs.

Don't forget we would need to know the type of battery.

I believe you just assumed lithium but op did not specify.

 

[OffGridInTheCity]

Don't forget we would need to know the type of battery.

I believe you just assumed lithium but op did not specify.

Good point. You don't want to discharge a battery lower than it's specifications.

If it's lead-acid, you probably will not have a BMS and so you'll have to monitor to make sure you don't discharge too far. I would look for specs but as a rule of thumb from what I've read I wouldn't go below 50% DOD on any lead-acid.

If you have a lithium based battery such as LifePo4, you should have a BMS and may be able to depend on a low-voltage cut-off, but of course if you don't know or have not tested the BMS I would monitor and be ready 'manually' to stop the test so that the voltage does not go below the specs of the battery.

 

[oshky]

Thank you for your advice!