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Old 21-07-2019, 00:04   #1
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When to stop charging my Lifepo4 bank

I have now been using my new lifepo4 bank( 8x100A = 200ah winston+ 123Bms) for the last 4 weeks of full time cruising. Works very well and I have had plenty of power the whole time. We usually move every other or third day and motor maybe 1-2 hours. Between that I have 150w of solar that have been supplying 3-7A during daytime. I have hooked the charging up via a relay that either is controlled by the BMS or manually via a switch. My problem is if I let the BMS controll the stop charging when motoring it shuts the charging of after like 10 minutes because then the cell voltages are up over my balancing voltage (set to 3,40). The BMS interpret this as 100% soc and shut the relay off. But I know that I might have had 20% Soc when starting and there is no way the bank is full after just 5-10 minutes. The alt gives like 65A via a Sterling pro alt set to lifrpo4. After another 10 minutes after the charging have stopped the cell voltages are down to like 3.28 or similar again. If I controll the stop charging manually and let it continue then after like 2 hrs the alt is dialed down by the Sterling to like 5-6A and then the consumption is usually higher so the charging stops by “itself” and the cell voltages slowly drop below 3.4 again.. Being on the hook I obviously want the bank to charge as full as possible when doing my short motoring. So how should I think in this matter? Should I raise the balancing voltage to 3.5? Or just controll the charging manually?
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Old 21-07-2019, 00:46   #2
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Re: When to stop charging my Lifepo4 bank

You won't get a consensus on this.

Here's my take - step 0 is, you need to decide how high **you** want to define your

"daily usage cycling 100% Full SoC" (aka level D)

as opposed to the batt mfg

"theoretical / vendor 100% Full SoC" (aka level B)


If you are trying to define D as a CC-only no-CV profile, then the setpoint will need to vary, depending on the C-rate of the charge current.

Measure voltage very precisely after a couple hours isolated resting, use that as an objective proxy for SoC,

as opposed to the more usual

"hold Absorb/CV at X voltage until endAmps (trailing amps declines to Y C-rate)"

which gets to the same SoC point every time regardless of a low 0.05C or a high 0.35C current rate.

At high charge rates vs low ones, the same stop-charge voltage (CC-only no-CV) will deliver different SoC results, higher the rate lower the SoC.

At super low charge currents, below the usual endAmps range say .005C, there is a danger of overcharge even holding voltage at 3.4Vpc. Our Cpt Pat recommends using an Ah-counter stop-charge approach in that scenario.

Also, in case this isn't complicated enough, some users have experienced a "memory effect", reduced capacity from repeatedly stop-charging at the same level D, earlier/lower-SoC than the vendor level B spec, never getting to the latter for many months and years.

It is recommended by some to therefore either go to level B every time, or,

my reco is to do so once or twice a month, or every 20-25 level-D cycles if not cycling every day.

______
Next, I would not use a single layer of defence BMS to handle charge control.

IMO there needs to be a failsafe layer of HVC protection in place for when the normal charge regulator fails.

Of course, **if** you rely on your BMS to handle balancing, and it only does so in the last stage of charging, you need to ensure that there is enough time to get the balancing finished, given the fact that the usual balancing current available is very very low and slow, often under 1A.

Or occasionally bring the cells back to perfect balance using other equipment, manually or otherwise.
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