Ideal Drop-in Lifepo4 Battery Purchase Strategy

[Adelie]

Originally Posted by SOLAR SUPPORT

Why should we accept your claims that the reports based on information and experiments published by brands that people, including me, are highly trusted, do not reflect the truth, and that these are publications for advertising purposes? ....

My experience has shown that business entities will occasionally present info that supports their product and gloss over or not present info that does not support their product or is adverse to the sail of their product. Occasionally they even outright lie.

An example of this is the petroleum industry's long standing denial that use of fossil fuels is leading to climate change.

[smac999]

Originally Posted by wholybee

You make a good point, there is still a lot of Wh in a LA battery at 50%. But I'm not actually sure the LA will get me home safe. Yes, the LA still has power left. But is it all USABLE? The lead acid battery drops in voltage significantly. That you still have 40%-50% of capacity left doesn't matter if your radio and Navigation equipment shuts off because the voltage is too low. With no load, a 50% LA battery is already at 12v. With even a small load, it is below 12v. Some stuff will still work. LED lights will. VHF probably will in RX mode, but in transmit might pull the voltage down too much. If you are running a laptop off an inverter for navigation? Forget it.

The reality is that in the example you gave, you probably would get the same full (meaning all your equipment works) working time out of both. Although the LA will keep the lights on after your navigation gear keeps working until it gets to 0%.

Lots of stuff will work down to 10v. I tested a b&g plotter last year trying to trouble shoot why it was randomly shutting down. And it ended up worked down to 8 or 9v. The problem was something else.

[Pizzazz]

I would like to add/emphasize a couple of points regarding lead acid battery capacity.

1. DoD (depth of discharge) does not change the lifetime value of a lead acid battery significantly. If you discharge the battery by 80%, it will last fewer cycles but each cycle will give you 80% of the rated capacity in useable energy which is much more than shallow cycling to 15% DoD for example. I looked up the data from a reputable local producer (attached) and I get the following for 100Ah deep cycle AGM battery.

99% DoD, 280 cycles, 340 kWh lifetime energy (11% worse)
80% DoD, 400 cycles, 380 kWh lifetime energy (0% worse)
50% DoD, 640 cycles, 380 kWh lifetime energy (target)
30% DoD, 1200 cycles, 430 kWh lifetime energy (13% better)
15% DoD, 1600 cycles, 290 kWh lifetime energy (24% worse!)

The data shows that if you get a deep cycle AGM battery you need to work it, to at least 30% discharge to maximize its life and if you happen to discharge it to 50 or 80% on a regular basis, you will only lose 13% over the lifetime. You will have to replace the batteries more often (400 cycles vs. 1200 cycles) but it is just cost distributed over time vs. upfront.

The point above cannot be emphasized enough. It is relatively easy to design a boat to support voltage down to 10.5 or 9.5V (DC-DC converter for the electronics, change the low voltage cut-off of the inverter, lights, pumps, fridge have no issues... even if the fridge cuts off due to low voltage, it will resume once the higher load is removed). Then, if you purchase minimum size AGMs and target 80% DoD, one is quite ahead as lithium becomes 2x more expensive.

2. The second point is that insufficient charging is a far bigger problem on cruising boats than capacity and discharge rates. I just accept it, do not think about it and aim to replace the batteries more often because under charging kills them. It is just not possible to charge AGMs away from the marina unless you overinvest in solar.

[SOLAR SUPPORT]

I know that there are many boat owners who are considering going with LA service batteries. I was thinking the same way a year ago, and I agree with you on that. However, my AGM batteries were not of very good quality. Even though I rarely discharged them below 12.0 volts, in their 5th year their capacity had dropped considerably. I decided on LFP, which is a bit risky but will give me future advantages. If you are making the LA-LFP transition with your own means and electrical skills, you can keep the transition cost at a reasonable level by establishing a simple and secure infrastructure. If you get professional help and use advanced tracking mechanisms and new chargers, you will encounter a cost well above the cost of the LFP battery. In this case, it may be a more reasonable and logical decision to continue with LA batteries. Not taking risks is also a sensible option. Praising LFP technology or criticizing LA batteries or comparing these technologies is a pointless discussion for me.

[s/v Jedi]

Originally Posted by Pizzazz

I would like to add/emphasize a couple of points regarding lead acid battery capacity.

1. DoD (depth of discharge) does not change the lifetime value of a lead acid battery significantly. If you discharge the battery by 80%, it will last fewer cycles but each cycle will give you 80% of the rated capacity in useable energy which is much more than shallow cycling to 15% DoD for example. I looked up the data from a reputable local producer (attached) and I get the following for 100Ah deep cycle AGM battery.

99% DoD, 280 cycles, 340 kWh lifetime energy (11% worse)
80% DoD, 400 cycles, 380 kWh lifetime energy (0% worse)
50% DoD, 640 cycles, 380 kWh lifetime energy (target)
30% DoD, 1200 cycles, 430 kWh lifetime energy (13% better)
15% DoD, 1600 cycles, 290 kWh lifetime energy (24% worse!)

The data shows that if you get a deep cycle AGM battery you need to work it, to at least 30% discharge to maximize its life and if you happen to discharge it to 50 or 80% on a regular basis, you will only lose 13% over the lifetime. You will have to replace the batteries more often (400 cycles vs. 1200 cycles) but it is just cost distributed over time vs. upfront.

The point above cannot be emphasized enough. It is relatively easy to design a boat to support voltage down to 10.5 or 9.5V (DC-DC converter for the electronics, change the low voltage cut-off of the inverter, lights, pumps, fridge have no issues... even if the fridge cuts off due to low voltage, it will resume once the higher load is removed). Then, if you purchase minimum size AGMs and target 80% DoD, one is quite ahead as lithium becomes 2x more expensive.

2. The second point is that insufficient charging is a far bigger problem on cruising boats than capacity and discharge rates. I just accept it, do not think about it and aim to replace the batteries more often because under charging kills them. It is just not possible to charge AGMs away from the marina unless you overinvest in solar.

Now we’re getting to the meat of it

So, assuming a daily charge cycle (solar), one had to drastically reduce the installed capacity of the LA bank in order to make such deep cycles, then replace them every other year.

Not sure if this is better than LFP… but it will be cheaper.