AGM Battery - normal discharge rate

[jordanbigel]

I had a conversation with a neighbor this morning, he's ex-navy, getting a boat ready to cruise, he seems pretty smart. But, the subject came up of power usage and charging, and he stated that his new AGM batteries could be routinely discharged down to 20% charge (80% discharge) without harm. I told him I didn't think that was right, and maybe they could do that once or twice without affecting longevity, but if you do that regularly while cruising you will kill your batteries pretty quickly (2-3 years I would guess).

And I see a lot of battery sales-seak online claiming the DoD is 80%, I read at least one "article" claiming that you can go down 80% because you end up with fewer charging cycles - I don't buy that. My understanding was that the strength of AGM batteries is faster acceptance rate when charging, and higher max discharge amps (ability to draw more power faster) - but any LA battery will suffer if discharged to 80% routinely, right? Or not?

[smac999]

The fireflys are rated for 20%

They still have voltage drop though. So at 20% some stuff may stop working as the voltage will be low.

Most agms are probably 40%

[lmxr]

Originally Posted by jordanbigel

I had a conversation with a neighbor this morning, he's ex-navy, getting a boat ready to cruise, he seems pretty smart. But, the subject came up of power usage and charging, and he stated that his new AGM batteries could be routinely discharged down to 20% charge (80% discharge) without harm. [...]

Any discharge, and especially keeping the battery in a discharged state, damages a lead acid battery. The deeper the discharge, the larger the harm. While some batteries handle this better than others, the relation between depth of discharge (DoD) and achievable cycles is always continuous and there is no DoD 'threshold' under which 'no harm' is done.

A quality battery comes with a datasheet that shows the number of achievable cycles for several DoD. It should also document the testing methodology. Take for example this excerpt from the Victron SuperCycle AGM datasheet (https://www.victronenergy.com/upload...ttery-EN.pdf):

≥ 300 cycles @ 100% DoD (discharge to 10,8V with I = 0,2C₂₀, followed by approximately two hours rest in discharged condition,
and then a recharge with I = 0,2C₂₀)
≥ 700 cycles @ 60% DoD (discharge during three hours with I = 0,2C₂₀, immediately followed by recharge at I = 0,2C₂₀)
≥ 1000 cycles @ 40% DoD (discharge during two hours with I = 0,2C₂₀, immediately followed by recharge at I = 0,2C₂₀)

Note that in the two latter tests the battery was immediately charged after being discharged. This reduces sulfation damage. Had the battery been left for a few hours in partially discharged state (as is common on sailing vessels), there would be more damage and the number of cycles would be less.

To which DoD you operate the battery is your choice. In practice you'll never get the number of cycles written in the datasheet unless you strictly adhere to the discharge and charge characteristics of the cycle test (close to impossible on a sailing boat...).

For more discussion, I think the following article explains it pretty well: https://marinehowto.com/what-is-a-deep-cycle-battery/. While it is written primarily about flooded batteries, the cycle life vs. depth of discharge discussion applies to AGMs as well.

[ranger58sb]

Originally Posted by jordanbigel

I had a conversation with a neighbor this morning, he's ex-navy, getting a boat ready to cruise, he seems pretty smart. But, the subject came up of power usage and charging, and he stated that his new AGM batteries could be routinely discharged down to 20% charge (80% discharge) without harm. I told him I didn't think that was right, and maybe they could do that once or twice without affecting longevity, but if you do that regularly while cruising you will kill your batteries pretty quickly (2-3 years I would guess).

And I see a lot of battery sales-seak online claiming the DoD is 80%, I read at least one "article" claiming that you can go down 80% because you end up with fewer charging cycles - I don't buy that. My understanding was that the strength of AGM batteries is faster acceptance rate when charging, and higher max discharge amps (ability to draw more power faster) - but any LA battery will suffer if discharged to 80% routinely, right? Or not?

Odyssey (and probably others) advertise their typical DoD as 80%, but Firefly Oasis are the only AGMs I know of where that's actually (probably) OK.

I think Odyssey (et al) are maybe chasing military, off-road, and powersports sales with that 80% figure, in applications where longevity isn't the most important criterion.

-Chris

[ikanode]

[QUOTE=jordanbigel;3494641I read at least one "article" claiming that you can go down 80% because you end up with fewer charging cycles[/QUOTE]

That's at least partly true depending on circumstances if you focus on lifetime AmpHours out of the battery. There's not all that much difference in total AmpHours output.
50% = longer life, more cycles, but less output per cycle
80% = shorter life, less cycles, but more output per cycle

What's the goal? Is the focus on longest calendar life or to also give consideration to AmpHour capacity per cycle. A focus on AmpHours per cycle may let you use smaller cheaper batteries but need to replace them more often.

Again, at least partly true. But the difference probably isn't as big as commonly understood. Talk to your battery supplier and see if they have any data on their batteries.

[jordanbigel]

Originally Posted by ikanode

What's the goal? Is the focus on longest calendar life or to also give consideration to AmpHour capacity per cycle. A focus on AmpHours per cycle may let you use smaller cheaper batteries but need to replace them more often.

Well, as I said a the top - this really is about "a friend" - my neighbor, 2 slips away, who is getting his boat ready for long term cruising. I am pretty sure his priorities are economy -- ie. making his batteries last as long as possible is almost certainly his goal. Which is why I told him I thought regular discharging of 80% of his batteries capacity would guarantee shorter lifetime of the batteries, and from what I have read here and confirmed by reading several manufactures specifications, I believe I am correct. Even optima and firefly specs show that regular cycling to 80% will reduce the total number of cycles, and hence the number of years before replacement.

My boat has gel cells and I never discharge below 50%, and usually try to keep to 30-40% before recharging, but he seemed pretty certain his new AGM bank were different. I have not seen him again to ask him what brand, but I think the consensus is that this holds true for all types of SLA batteries, be they gels, AGMs, or whatever. Deeper regular discharge leads to shorter longevity.

[Adelie]

Firefly longevity is less affected by deep discharge but it is still affected.

What’s going to burn him more is the inability to charge at 0.2C minimum.
0.3-0.4C is better.

If he has a large bank or a small alternator that will get him just as fast or faster.

[powerpak]

My battery management system only gives me battery voltage, not DoD. How do you determine DOD?

[Sailmonkey]

Originally Posted by powerpak

My battery management system only gives me battery voltage, not DoD. How do you determine DOD?

With an amp hour totalizing counter.

[powerpak]

Thanks Sailmonkey

[Adelie]

Originally Posted by powerpak

My battery management system only gives me battery voltage, not DoD. How do you determine DOD?

If you have characterized your battery bank you can estimate from voltage.
With FLAs you could characterize it from specific gravities.
With AGMs you will need to get an energy meter that has a shunt and can count Ahr like this:
https://www.amazon.com/Digital-Multi...390915&sr=8-28

Fully charge the battery, Turn on a load drawing 0.02C. When you get to 90%SoC per the Ahr counter record the voltage. Continue measuring at 80%, 70%, 60%, 50% and if you are a little daring at 40%. Graph SoC against the voltage. Repeat for loads of 0.05C, 0.10C, 0.15C, 0.25C graphing each separately. I would probably only go to 40% for 0.05C & 0.10C

Yes this is a multi-day process probably done at the dock using a regular charger to recharge between tests, and you have to have the solar panels and any other charging source disconnected while loads are being applied.

Here's some links on this: https://marinehowto.com/under-load-b...oltage-vs-soc/
https://workshoppist.com/agm-battery-state-of-charge/

And here's an example of the results: https://www.arttec.net/Solar_Mower/4...20Charging.pdf

The graph below is for batteries of an undefined type. You want to create a graph specific to your battery bank. With the Amps reading you know which curve to look at, from that using the voltage you can estimate SoC. If your amps are between curves, eye ball it.

[BillKny]

Originally Posted by smac999

The fireflys are rated for 20%

They still have voltage drop though. So at 20% some stuff may stop working as the voltage will be low.

This is WRONG. There is very little voltage drop with Fireflies until they are nearly empty.

We have done a number of full discharge tests of a large Firefly bank at a C10 discharge rate. When full, and discharging at a C10 rate, the batteries are at 12.53 Volts. When they have dropped to 80% DOD (while still drawing at the C10 rate!) the voltage is at 11.60V.

Can you name a SINGLE piece of equipment that will shut down at 11.6 volts? Our inverter, for example, doesn't shut down on low voltage unto 10.8V. Our refigerators keep running to 10.5V.

Too be fair, it is about 83% or 85% where the voltage begins to drop more quickly. We consider 10.8V while at a C10 discharge rate to be "100% DOD"

[powerpak]

Thanks Billkny, much appreciated, powerpak

[smac999]

Originally Posted by BillKny

This is WRONG. There is very little voltage drop with Fireflies until they are nearly empty.

We have done a number of full discharge tests of a large Firefly bank at a C10 discharge rate. When full, and discharging at a C10 rate, the batteries are at 12.53 Volts. When they have dropped to 80% DOD (while still drawing at the C10 rate!) the voltage is at 11.60V.

Can you name a SINGLE piece of equipment that will shut down at 11.6 volts? Our inverter, for example, doesn't shut down on low voltage unto 10.8V. Our refigerators keep running to 10.5V.

Too be fair, it is about 83% or 85% where the voltage begins to drop more quickly. We consider 10.8V while at a C10 discharge rate to be "100% DOD"

Pulling ~150a (microwave) to in inverter is going to drop 0.5-1v beteeen battery and inverter even with 4/0 cable.

And the batteries are going to drop 0.5v as well with that load. So your 11.6v battery at rest is now 10v at the inverter with a big load

[Adelie]

Voltage drop for wiring is going to depend on length of run alas well as amperage and cable size.

Battery voltage drop will depend on battery bank size, SoC, and battery health as well as amperage.