AGM: Depth of Discharge

[Letterkenny]

Read a couple arguments on other forums about discharging AGM below 50% and whether it would be worth it. Seemed like the data presented suggested a 6% drop in life of the battery going down to 20% remaining capacity versus 50%. Has on this forum considered this at all and can provide some insight?

I have a new boat but insufficient battery bank size. I can either add AGMs to it or go lithium but if I can get to 80% DOD on the AGMs over the next 5 years, then I’d likely go that route and see what has developed on the LIFEPO4 front.

[Paul L]

Find the specs from your battery's manufacturer. Here's an example from Lifeline

[sailingharry]

One word of caution on that diagram that Paul provided. It's a great diagram, and the kind of thing us engineers love to look at. But, lay folks beware -- it is plotted on semi-log paper. This makes the curve look almost flat, and can lead you to think that there isn't much difference between 20, 40, and 60% DoD. But look carefully at the Y axis. At 20% DoD, you get 3000 cycles. At 40% DoD, you get 1500 cycles (half as many). At 60% DoD, you get 800 cycles (half as many again). And at 80% DoD, you get a mere 500 cycles (about another half). It's the magic of semi-log paper -- nothing wrong, just read the scales.



But, look at it another way. If you routinely discharge to 80%, you could replace the battery in 500 cycles. Or you could double the size of your battery (twice the cost) and discharge to 40% -- and get 1500 cycles. Then for twice the price and twice the space, you get 3 times the service. Yes, a better value for your budget. But is it really a better "value?" Depends on your use, your space, etc -- and on a 34' boat, space is worth a lot! And if you only use the boat 50 days a year, 500 cycles is 10 years (of course, they will "just die" long before that!).

[more]

Originally Posted by Letterkenny

Read a couple arguments on other forums about discharging AGM below 50% and whether it would be worth it. Seemed like the data presented suggested a 6% drop in life of the battery going down to 20% remaining capacity versus 50%. Has on this forum considered this at all and can provide some insight?

I have a new boat but insufficient battery bank size. I can either add AGMs to it or go lithium but if I can get to 80% DOD on the AGMs over the next 5 years, then I’d likely go that route and see what has developed on the LIFEPO4 front.

In reality, all AGM battery is a scam. if you buy 100 ah battery this battery maybe have 80-90 ah. on C20.
Always is better normal stationery lead battery.
very soon you learn.

[GreenWave]

Originally Posted by sailingharry

If you routinely discharge to 80%, you could replace the battery in 500 cycles. Or you could double the size of your battery (twice the cost) and discharge to 40% -- and get 1500 cycles. Then for twice the price and twice the space, you get 3 times the service. Yes, a better value for your budget. But is it really a better "value?" Depends on your use, your space, etc -- and on a 34' boat, space is worth a lot! And if you only use the boat 50 days a year, 500 cycles is 10 years (of course, they will "just die" long before that!).

Harry is spot on here... While number of cycles sounds great, if that takes you out 8+ years based on use then you are not going to get the full value of the AGM purchase based on cycles. Of course, other factors include where you are using them (warmer temps probably eat batteries quicker) though cold temps won't allow full charge holding.

[Letterkenny]

Thanks all. Sort of what I was expecting. There’s more value to me to discharge deeper and replace more often than having a larger bank or recharging more often. I’m in San Diego so pretty ideal temps for these.

Any concerns mixing in 2x West Marine group 24 79ah AGM dual purpose with my existing 70ah AGM Exide dual purpose batteries? Age and cycle wise, it’s close enough as my boat was delivered in August. I’m just debating if I want to repurpose the start battery and use one of the WM as the new start or add in a second battery to double the capacity.

[JBP]

Originally Posted by Letterkenny

Thanks all. Sort of what I was expecting. There’s more value to me to discharge deeper and replace more often than having a larger bank or recharging more often. I’m in San Diego so pretty ideal temps for these.

If you are prepared to use them up with deep discharge and replace them frequently, why bother with AGM? Ordinary FLA deep cycle batteries are dirt cheap compared to AGM. By the time you burn through a couple sets of AGM, it would be cheaper just to go lithium.

[thinwater]

Perhaps the greatest weakness of the 80% DOD plan" is the lack of safety factor. We design for 50% DOD, but once in a while we go lower than that, no big problem. But if you base your design on 80% DOD, you will go dead flat once in a while. At 80% DOD, may electronics will stop functioning (low volts); some will stop functioning at 50% DOD in cold weather. And as the batteries age, 80% DOD becomes 100% DOD.


So we design for 50% DOD to cover many contingencies and to insure long functional life. You always design equipment to remain functional through the expected useful life, and that generally requires a safety factor when new.

[mako]

And our interpretation of that chart is even more theoretical because the OP needs to ask himself “what his use profile is?”

Does he discharge every single day to 80% DOD and then charge up fully then repeat the next day? If so then his batteries will only last 500 days or maybe a couple of years.

But a few solar panels and the reality of a random, mixed DOD profile, and likely that battery bank will last 4-5 years.

Then goto Costco and for $130 a pop just replace and repeat all over again.

[Letterkenny]

While I’d love to be a full time cruiser, I usually use my boat twice a week. Will do some week trips. I plan to install some temporary solar panels with magnets. Honestly plan is more overkill but I get battery anxiety.

For the question why not flooded, well my boat came with AGM which are pretty new. Further, thought I was told that they can get only like 30 cycles if much further cycled down.

[sailingharry]

If you "use your boat twice a week" it doesn't sound like you normally discharge very deep -- it sounds more like day sails. The week trips, and weekends, are where you get deeper discharge. That leads more toward approving of the occasional deeper discharge.


But Thin has a good point. And disagreeing with Thin is like disagreeing with MaineSail -- "don't." LOL. But as long as you have a start battery (or a "reserve" battery), you have little risk. As he points out, you may start to see evidence of failing batteries -- but when you get there, you replace them. Unless you are part way through a summer-long cruise, the inconvenience factor is low.


Battery anxiety? I have 800Ah of AGM, have a daily budget of around 200Ah, can charge at over 200A, and this summer (110 days with 4 days of shore hookup), I suffered from daily "battery anxiety" -- just ask my wife! LOL

[Letterkenny]

Like I said before, I also plan to add some solar (likely 200 watts) that I’d mount on the Bimini or dodger on longer trips and I also have both a portable generator and a lithium external battery to use the AC outlets when the generator isn’t on board. So, between the alternator, generator, and solar, I don’t think I run much risk of going low as I’ll at least have one of those backups when I’d discharge more.

Sounds like I more so should just add 1 AGM as the new start battery and then re-wire my old start battery as an additional house as they are identical batteries.

[more]

Originally Posted by Letterkenny

Sounds like I more so should just add 1 AGM as the new start battery and then re-wire my old start battery as an additional house as they are identical batteries.

or add this
https://www.aliexpress.com/item/1005...383d63e31Y2i6y

this is a little more of your 70ah agm battery and in half price of your agm

[citlab]

Originally Posted by Letterkenny

Read a couple arguments on other forums about discharging AGM below 50% and whether it would be worth it. Seemed like the data presented suggested a 6% drop in life of the battery going down to 20% remaining capacity versus 50%. Has on this forum considered this at all and can provide some insight?

I have a new boat but insufficient battery bank size. I can either add AGMs to it or go lithium but if I can get to 80% DOD on the AGMs over the next 5 years, then I’d likely go that route and see what has developed on the LIFEPO4 front.

In am scientifically inclined by profession, but on boat matters I prefer pragmatics over theory.



We bought our boat as new and and ran through 2 sets of 2xFLA 110Ah house batteries that both failed terminally at 2.5 semester. After the 5 first years we changed from 2x FLAs to 3x AGM 110Ah. We always run the batteries to "death" should we need the capacity. The 3x AGM have now run from 2010 without any detectable degradation in capacity and we do continue to get 5 days of autonomy.


For longer autonomy needs, I'd follow the good advice available in this forum.


That's my two cents of insight.

[Uncle Bob]

Originally Posted by citlab

In am scientifically inclined by profession, but on boat matters I prefer pragmatics over theory.



We bought our boat as new and and ran through 2 sets of 2xFLA 110Ah house batteries that both failed terminally at 2.5 semester. After the 5 first years we changed from 2x FLAs to 3x AGM 110Ah. We always run the batteries to "death" should we need the capacity. The 3x AGM have now run from 2010 without any detectable degradation in capacity and we do continue to get 5 days of autonomy.


For longer autonomy needs, I'd follow the good advice available in this forum.


That's my two cents of insight.

One needs to look at the actual fla battery type, not all fla batteries are created equal.