A cardinal sin???

[Tracyandkimo]

We will be converting to Lithium in May but are our AGMs are no longer making it through the night so we need a temporary fix. I was considering adding a sacrificial new AGM to our bank to hold us over. The + & - leads are off of opposite ends of the bank, which I hope would lessen the load on the new battery. I would approximate the voltages before connecting. Other than premature aging of the new battery, what other concerns are there? No haters w worthless comments please. Thanks

[Tin Tin]

A few questions first:
What is size of your house bank?
What do you run all night that flattens the batteries?
Are your start batteries in good condition - are they AGMs' too?
Do you have a generator of any kind?
Do you have solar panels?
Where are you?

[Tracyandkimo]

We’re in the Caribbean until May. Our house bank is 840ah. Separate starter batteries that are not AGMs. . Refrigeration (2 freezers/2 fridges) is our big night time draw and can be as high as 25-30ah. . We have a lot solar (2,760w) but this time of yr it doesn’t really add surplus until 7:30-8am, then quickly brings SOC up to 80% at which time it trickles in up to low 90’s before the sun’s angle decreases and we’re back to a negative draw after 4:30. In summer time we get positive input from 7am-6:30pm, which is part of the discrepancy. We just don’t have the capacity to store all the power the panels are producing mid day.
To stay above 50% SOC, at night I run alternator (no need to run genset when AGMs won’t take much charge at this point) to bring them above 80% before bedtime and then run Genset/100 amp charger around 4:30-5am to keep them above 50% while waiting for sun. I just need a short term patch to get me to May. The price of diesel saved charging would pay for the sacrificial AGM battery!

[Tortuga's Lie]

I think the big thing is the premature death of the new battery, as you already know but how long this will work is a big question mark. How old are your current AGM's? If they are less than 8 years old, there could be something going on that is killing them quicker.
Can you pair down your refrigeration and run on 1 fridge and 1 freezer? This would help the situation but I am sure you've considered this already as well.

[Tupaia]

Perhaps creating a separate system with a new battery 100-110ah and divert some solar to this to supply the fridge / freezer scenario. What about switching 1 x fridge and 1 x freezer off.

[Tracyandkimo]

I believe it’s the daily cycling from 50-90% that’s aging them

[Tracyandkimo]

Throwing out all the vacuum packed food would be more expensive than the battery LOL

[sailorboy1]

Originally Posted by Tracyandkimo

We will be converting to Lithium in May but are our AGMs are no longer making it through the night so we need a temporary fix. I was considering adding a sacrificial new AGM to our bank to hold us over. The + & - leads are off of opposite ends of the bank, which I hope would lessen the load on the new battery. I would approximate the voltages before connecting. Other than premature aging of the new battery, what other concerns are there? No haters w worthless comments please. Thanks

As long as the old batteries don't just drain the new one I see no problem.

Maybe instead of adding a new battery to an old bank it would be better to get a couple new ones and just replace the bad batteries. You have to decide if the cost of a couple of batteries is worth having the power to you.

[Tin Tin]

When you get up in the morning before starting the generator what voltage is the battery monitor showing, and then what does it show when you switch everything off including inverter.

[thinwater]

Originally Posted by Tracyandkimo

Throwing out all the vacuum packed food would be more expensive than the battery LOL

Change your diet. You really do NOT need all that refrigerated food. Obviously. Eat what you have and stop buying perishables you won't eat quickly.

[SailingHarmonie]

How are you measuring the SOC? If by a standard battery monitor the reported number does not reflect a decline in the batteries, but rather an increase in usage, or in accuracy in the meter, especially if you have not been fully charged for many cycles.

You MIGHT have a battery problem, but you can not use the SOC meter number to tell you that.

[sailorboy1]

Just a note; when my batteries went bad last Dec and the freezer/frig would not run at night the food didn't go bad. The frozen food stayed frozen in the freezer and the perishables were fine at night as no one was opening the lids. I started just turning off the freezer/frig at night because every time they tried to start the fan next to my head would slow down and then speed back up when the freezer staup failed keeping me awake. During the day when they are really running on solar turn down the temps and start with lower temps and it will be fine.

I found charging the bad batteries a waste of time really as they just ran back down quickly. But during the day the solar would run the systems and "charge" a little.

You should separate your batteries and find the worst one/ones and remove from the system as they are a load really.

In the big picture a couple of replacement batteries are probably worth the cost for 6 months use till switching.

[wholybee]

Ok, some reality here. When new, your 840Ah bank was only good for 400Ah. If it's toast, you are probably only getting 200Ah, maybe even less. A 100Ah AGM will only have 50Ah of usable power, and in combination with a dead bank, might not even provide that.

You should test your bank, each battery individually. You might find a battery or two with a dead cell. In which case, removing the bad battery will improve the situation.

You need to know what your daily usage is, and how much of that is overnight, when presumably everything is off except your freezer. And you need to know what your current capacity is, and how much it has degraded.

With LFP, you get the actual sticker capacity, or at least very close to it. There is very little price difference between LFP and AGM right now. It is likely you could replace your entire bank with 200Ah of LFP, and have enough to get you through the night. A 400Ah bank would be substantially better than the 800Ah you started with.

If there is absolutely no budget for converting to a smaller LFP bank, then consider adding a single LFP to your existing bank. This is very much not recommended. But if it is only for a of months to hold you over, a single 100Ah LFP will make a substantial difference when a 100Ah AGM might not, and you could use the LFP later when you do the full conversion.

[sailorboy1]

batteries scheduled to be replaced in 6 months can most certainly be taken down to less than 50% regularly

[wholybee]

Originally Posted by sailorboy1

batteries scheduled to be replaced in 6 months can most certainly be taken down to less than 50% regularly

But they probably don't even have 50% left. That is, if you take that dead 800Ah bank down to 0% SOC, I bet you only get 200-300Ah out of it. Only taking it to 50%, maybe 100Ah, which is why I think even a really small LFP might be better than what is there now.

However, measuring it is the only way to know. Anything else is a guess.