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What Can I discharge Battry to While in Use??

Awskmet

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Jul 1, 2020
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Hey, so I've been testing my solar system in the past couple weeks and have an issue. While using my batteries (24 volt system), every once in a while my charge controller will warn me of an over discharge warning, even though there is plenty of charge left in the batteries. I have four 6 volt trojan-150 225 AH batteries in series (24 volts, flooded lead acid). I see it drops to 22 or so volts when I'm charging the laptop and the fridge compressor kicks on, assuming some kind of surge dropping the voltage too low that triggers the alert. I was hoping someone could tell me what's the deal? Is this common, and will this damage the batteries? I've also noticed that the next day, without any usage, the charge controller pulls in about 1.1 kilowatts throughout the day , roughly half the actual capacity of the batteries (assuming 50 percent depth of discharge). After some research , I am kind of just confused about the whole ordeal, and want to know if this was a normal thing I didn't expect to happen, as I am still buttoning up the whole system, with much help from this community. Please, help a brother out, thanks!
 
Does your charger make it all the way through to float? I'm wondering if your batteries are actually getting fully charged.

The fridge compressor will draw a fair whack of current when it starts, unless the fridge in an inverter type. That could be enough to make a dent in the battery voltage during start up but it should float back up a bit once the high draw stops, assuming your battery is getting fully charged. The charger warning is likely based just on battery voltage so for brief high current draws it's likely just false warning. The voltage of a lead acid battery can only be used as a state of charge gauge if the battery has been sitting with no load and no charging for several hours.

If you want to track the SOC of your batteries the best idea is to get a watt meter or a battery monitor similar to a Victron BMV. There's cheap meters on ebay etc that will keep track of the amp hours in and out of your battery too and they will give you a much better idea of where your batteries are compared to using the in-use voltage. Similar items can be found on ali*, amazon and pretty much everywhere else.

If you get a cheap meter make sure it can read current bi-directionally, ie going in and out otherwise it won't be fit for this purpose. Make sure you get one that can handle the maximum amps you draw +25%.

 
With that kind of capacity, I would not expect a fridge compressor kicking on and charging a laptop to pull the voltage down to 22V. That's dangerously low for a minor load. 1.1kWh is only about 20% of your capacity, so something is wrong.

I suspect that you're not fully charging your batteries, and getting them to 100% every day is critical to their health. Are you certain that the 1.1kWh represents a full charge?

Have you checked the specific gravity of the cells? That's the only way you can be certain the batteries are fully charged.

Have you read these two resources thoroughly and have a plan to maintain your batteries?


 
I took 1.1 kW to be the rate of charge across the day, not 1.1kWh for the day. Might be right, might be wrong. He has a notional 5.4kW of storage, half that is 2.7, half that is ~1.3 so maybe it is 1.1kWh. ?‍♂️
 
Does your charger make it all the way through to float? I'm wondering if your batteries are actually getting fully charged.

The fridge compressor will draw a fair whack of current when it starts, unless the fridge in an inverter type. That could be enough to make a dent in the battery voltage during start up but it should float back up a bit once the high draw stops, assuming your battery is getting fully charged. The charger warning is likely based just on battery voltage so for brief high current draws it's likely just false warning. The voltage of a lead acid battery can only be used as a state of charge gauge if the battery has been sitting with no load and no charging for several hours.

If you want to track the SOC of your batteries the best idea is to get a watt meter or a battery monitor similar to a Victron BMV. There's cheap meters on ebay etc that will keep track of the amp hours in and out of your battery too and they will give you a much better idea of where your batteries are compared to using the in-use voltage. Similar items can be found on ali*, amazon and pretty much everywhere else.

If you get a cheap meter make sure it can read current bi-directionally, ie going in and out otherwise it won't be fit for this purpose. Make sure you get one that can handle the maximum amps you draw +25%.

Thanks for the response. I have a battery meter installed, and each day it makes it up to at least 97 percent charge. I may not have been too clear in the previous post. The battery only drops that low for a couple seconds, triggering the warning, and then returns to a reasonable voltage. Is this quick drop dangerous? It will also do the same when the AC kicks on sometimes. Even though the batteries have plenty of charge, certain things drop the voltage pretty low which triggers the warning
 
I took 1.1 kW to be the rate of charge across the day, not 1.1kWh for the day. Might be right, might be wrong. He has a notional 5.4kW of storage, half that is 2.7, half that is ~1.3 so maybe it is 1.1kWh. ?‍♂️
I took 1.1 kW to be the rate of charge across the day, not 1.1kWh for the day. Might be right, might be wrong. He has a notional 5.4kW of storage, half that is 2.7, half that is ~1.3 so maybe it is 1.1kWh. ?‍♂️
1.1 was the total draw for the next day, but I don't think the batteries are empty, it's just that something is going on where the voltage drops low quickly
 
With that kind of capacity, I would not expect a fridge compressor kicking on and charging a laptop to pull the voltage down to 22V. That's dangerously low for a minor load. 1.1kWh is only about 20% of your capacity, so something is wrong.

I suspect that you're not fully charging your batteries, and getting them to 100% every day is critical to their health. Are you certain that the 1.1kWh represents a full charge?

Have you checked the specific gravity of the cells? That's the only way you can be certain the batteries are fully charged.

Have you read these two resources thoroughly and have a plan to maintain your batteries?


I would be very surprised if the batteries weren't fully charged, I have a little over 1200 watts of panels charging the batteries each day. The 1.1 is what it pulls in the next day, but I am not assuming that the batteries were fully drained the night before, only that the compressor kicking on dropped the voltage low enough to trigger the alarm briefly, and was wondering if this is normal/dangerous? I recenly installed a battery monitor, and it is bringing the batteries up to 97 percent each day (also strange because there is more than enough power to get the batteries up to 100 percent, I know this because for the last 3 or so hours of charging, anytime something is plugged in the charge controller draws in more to compensate for the added load)
 
Thanks for the response. I have a battery meter installed, and each day it makes it up to at least 97 percent charge. I may not have been too clear in the previous post. The battery only drops that low for a couple seconds, triggering the warning, and then returns to a reasonable voltage. Is this quick drop dangerous? It will also do the same when the AC kicks on sometimes. Even though the batteries have plenty of charge, certain things drop the voltage pretty low which triggers the warning

If it rises back up again it's probably fine. If you can check the peak amp draw your load puts on the battery, a peak hold DC clamp multimeter usually isn't fast enough but will give you a good idea. If you aren't looking at 50A or more you may find your batteries are not in good condition. You could do a discharge test on them to check their performance and work out their present amp hour rating and adjust your meter to suit.
 
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