Deep Cycle Batteries - Specs vs actual Ah

[p38arover]

In another thread https://www.aulro.com/afvb/alternate-energies/268200-mppt-pwm-solar-controller.html?268200 I commented on my 2 x 45Ah batteries in parallel struggling to keep up with my Engel 29 litre fridge.

When I looked more closely at the battery spec sheet this week, I noticed the 45Ah was at a cell voltage of 1.8 volts per cell (vpc), i.e., a battery terminal voltage of 10.8 volts. 10.5 volts is considered dead flat for AGM batteries.

Normally, low voltage cutouts are set to around 11.5 to 11.6 volts, i.e., 10-20% capacity remaining.

So I did a bit of searching of various deep cycle AGM battery manufacturers' website for battery specs. Virtually none of them specify the voltage at which the stated battery capacity is attained. Those that did, specified 1.75-1.8 vpc

So your 100Ah battery may only deliver 80Ah, maybe less.

A question thus arises - how low can one take a deep cycle AGM without damaging it permanently?

I've removed my batteries for bench testing. Preliminary tests with a voltage cutout set at 11.5 volts see my batteries delivering about 20Ah (measured on an eBay in-line watt meter). Recharging sees approximately the same power being put back into the battery.

For greater accuracy and graphing, I'll be setting up my MooshiMeter** Mooshimeter Australia (https://core-electronics.com.au/mooshimeter.html) on the batteries and running another discharge test. I'll post the results here.

** I used the MooshiMeter a while back to prove some solar panels or its controller I bought were faulty. I sent the supplier the graphs and they refunded my money. https://www.aulro.com/afvb/general-chat/36525-my-propensity-buying-faulty-goods-continues-83.html#post2768626

[goingbush]

Under-Load Battery Voltage vs. SOC – Marine How To (https://marinehowto.com/under-load-battery-voltage-vs-soc/)

[p38arover]

Thanks for that. A good read.

I was measuring voltage under load and at the battery terminals. During testing I was discharging just over 5A (approximately the 10 hour rate) as that's what my old Engel fridge can draw when working hard in hot weather.

The batteries have a rated capacity of 44Ah at 10hr, 1.8Vpc. Capacity at 40 deg C (the temp in my garage) is 102% and 100% at 25 deg C (77 deg F as mentioned in that article). The spec sheet doesn't show battery voltage cf. DOD that he recommends one should use.

My load was three 21 watt stop lights in parallel.

There is a lot of info on the 'net about AGM deep cycle batteries and a lot of it is contradictory and thus suspect. There's a lot of plagiarism, too.

[Homestar]

I work on around 70Ah usable for my 100Ah batteries, but in reality I've overkilled my system so don't need to take them anywhere near this - I have only ever seen my batteries drop below 12 volts once, and with my new setup I generally end up at 12.5 after running my inverter all night running a 40cm pedestal fan and the fridge, etc so I work on the idea of keeping my batteries as high as possible so they will last as long as possible. Given that 2 are 5 years old and the other 2 are older again, I'm happy with how I look after mine. I also know I have heaps up my sleeve should I need it.

[p38arover]

Unfortunately, I can't get a 100Ah battery under the floor in the L322 (under the panel where the rear fusebox/relay panel is located. Absolute maximum height is just under 200mm).

[Blknight.aus]

dont forget that in parallel peukert's law is against you if the batteries are not equally capable.

[p38arover]

dont forget that in parallel peukert's law is against you if the batteries are not equally capable.

To be totally honest, I don't think I've ever heard of Peukert's Law.

My batteries are same brand, model, etc. and were bought at the same time so I hope they are equally capable.

Off to read about Peukert's Law.................

Peukert's law - Wikipedia (https://en.wikipedia.org/wiki/Peukert%27s_law)

................ Ah, I knew about some of that but not all. To plug some figures into the equation would require better monitoring than I am doing but it's now obvious that the actual capacity will be significantly less than the stated capacity.

This gives a good description: BatteryStuff Tools | Peukert’s Law - A Nerd’s Attempt to Explain Battery Capacity (https://www.batterystuff.com/kb/tools/peukert-s-law-a-nerds-attempt-to-explain-battery-capacity.html)

I'm running a discharge test now using the Mooshimeter.