I am just wondering what method is used to determine the condition of an AGM, or Gel deep cycle battery. How to determine the remaining amp hours of a battery.

I don't think your answer was too far off the mark when one goes to a dealer of batteries that just wants to sell you a new battery. Can remember I was looking at batteries and the salesman came out and said let me test your batteries and showed me the load test unit pictured. I told him not to go near our van batteries with that. Thats OK for starter batteries, but not for deep cycle.

I just took mine to Battery World & left it with them for a couple of days.  At no cost they did two discharge tests & gave me a print out of the results.

They explained the results to me & made no attempt to sell me a replacement.

Great honest service I thought.

This test is by no means exact but will give a ballpark answer for a 50 100 Ah battery. More importantly, if you keep the results and do it again say in 6 months it will give you a trend of how the battery is holding up. So here is the test.

Fully charge the battery.   Obtain a headlight bulb 55 or 60 Watt.   Remove the battery from charge and wait about an hour. Measure the battery volts. If it is below 12.7 volts  your battery is already on the way out. If it is 12.8 volts or more proceed to next steps.  Put the bulb in a safe place where it cant set fire to something or get touched because it will get really really really hot.  If the battery is vented make sure that at the next step you connect to the battery terminals before you connect to the bulb (ie connect and disconnect at the bulb end. Put a switch in the line if you can. This ensures you do not make sparks near the battery which could cause an explosion. Not an issue with sealed batteries which yours most likely are. Connect the bulb across the battery and note the time or start your stopwatch. Check the battery voltage often as it discharges.  When it gets to 11.5 volts disconnect the bulb and note the time or stop your stopwatch. It will take a while to cool the bulb so while it is cooling you can do the math.

A 55 Watt bulb draws about 4.5 Amps (give or take) and is reasonably stable over a wide voltage range). So battery Amp hours = 4.5 X Hours (to the nearest hour if you like as it is only ballpark.

e.g. 1 hr = 4.5 Ah , 10 Hrs = 45 Ah, 20 Hrs = 100 Ah.

Strictly speaking the battery is normally discharged at a rate that would have a good battery discharged in 10 Hrs (ie the 10 hr rate) so a 100 Ah battery should be discharged at 10 Amps not 4.5 but as noted this will get you in the ballpark. There are various load devices available to test more accurately but a bit of overkill for most people.

Have fun, dont burn yourself.

I use one of these battery testers. I test the AGM battery when new & fully charged & note the readings. Then over time i check how its going, mainly the internal resistance seems to increase with age.

Ian195 wrote:

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This test is by no means exact but will give a ballpark answer for a 50 100 Ah battery. More importantly, if you keep the results and do it again say in 6 months it will give you a trend of how the battery is holding up. So here is the test.

Fully charge the battery.   Obtain a headlight bulb 55 or 60 Watt.   Remove the battery from charge and wait about an hour. Measure the battery volts. If it is below 12.7 volts  your battery is already on the way out. If it is 12.8 volts or more proceed to next steps.  Put the bulb in a safe place where it cant set fire to something or get touched because it will get really really really hot.  If the battery is vented make sure that at the next step you connect to the battery terminals before you connect to the bulb (ie connect and disconnect at the bulb end. Put a switch in the line if you can. This ensures you do not make sparks near the battery which could cause an explosion. Not an issue with sealed batteries which yours most likely are. Connect the bulb across the battery and note the time or start your stopwatch. Check the battery voltage often as it discharges.  When it gets to 11.5 volts disconnect the bulb and note the time or stop your stopwatch. It will take a while to cool the bulb so while it is cooling you can do the math.

A 55 Watt bulb draws about 4.5 Amps (give or take) and is reasonably stable over a wide voltage range). So battery Amp hours = 4.5 X Hours (to the nearest hour if you like as it is only ballpark.

e.g. 1 hr = 4.5 Ah , 10 Hrs = 45 Ah, 20 Hrs = 100 Ah.

Strictly speaking the battery is normally discharged at a rate that would have a good battery discharged in 10 Hrs (ie the 10 hr rate) so a 100 Ah battery should be discharged at 10 Amps not 4.5 but as noted this will get you in the ballpark. There are various load devices available to test more accurately but a bit of overkill for most people.

Have fun, dont burn yourself.

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 This is how I have done it too. But I generally do not discharge it to the bitter end as that will shorten the life. If you monitor the voltage you can see how it is dropping, for say 0.2 volt intervals, and see if it is ok or not so good, then choose a suitable final voltage without killing it. 

I made a little wooden stand with a couple of headlight globes to test with some years back. I tested a used battery recently that I bought. I only tested it for 0.2 V drop and was happy it took as long as mine did at home so the sellers story held up. 

Jaahn 

My 20 amp Victron Bluetooth controller you have a load connection, so easy get the data on your phone. 

Bigger controllers you do need a separate battery monitor.

I have one of these.

https://www.aliexpress.com/item/1005005080837439.html?pdp_npi=2%40dis%21AUD%21AU%2427.12%21AU%2420.61%21%21%21%21%21%402103011516910615497725452e52be%2112000031568123967%21btf&_t=pvid:a72708e3-da6e-4191-b1e4-960d0481f8a1&afTraceInfo=1005005080837439__pc__pcBridgePPC__xxxxxx__1691061550&spm=a2g0o.ppclist.product.mainProduct

Tim