6 (2s3p) 100Ah AGM batteries connected to 24v inverter-charger

[dborn]

Hi, another newbie here.
I have a power backup system (no solar) using a 24vdc 6kW split-phase inverter charger and 6 x 100Ah 12v AGM batteries.
The batteries are currently connected using short 4/0 cables in between each battery post. The connections to the inverter are diagonal (farthest [+] to closest [-] ) to try and keep batteries as balanced as possible. My system only gets used when there is a power outage and gets recharged back to 100% as soon as power is back.
Should I invest in busbars and active balancers to make my batteries last as long as possible or is my current setup with separate cables good enough?

I am considering buying tinned copper 30mm x 5mm bars on Amazon to build my own busbars to connect each battery directly to them, to parallel connect 3 batteries in series with another set of 3 parallel batteries. I'm thinking this would keep all batteries as well balanced to each other as possible, charging and discharging. Perhaps adding an active battery balancer to balance each set of 3 batteries with each other.
Useful? waste of money?
My 6 batteries would mostly go from 100% down to 30% and then back to 100% perhaps 5-6 times a year as this is a "whole-house UPS" that I manually switch on and off when it is needed.

Thanks

 

[sunshine_eggo]

Hi, another newbie here.
I have a power backup system (no solar) using a 24vdc 6kW split-phase inverter charger and 6 x 100Ah 12v AGM batteries.
The batteries are currently connected using short 4/0 cables in between each battery post. The connections to the inverter are diagonal (farthest [+] to closest [-] ) to try and keep batteries as balanced as possible. My system only gets used when there is a power outage and gets recharged back to 100% as soon as power is back.
Should I invest in busbars and active balancers to make my batteries last as long as possible or is my current setup with separate cables good enough?

I am considering buying tinned copper 30mm x 5mm bars on Amazon to build my own busbars to connect each battery directly to them, to parallel connect 3 batteries in series with another set of 3 parallel batteries. I'm thinking this would keep all batteries as well balanced to each other as possible, charging and discharging. Perhaps adding an active battery balancer to balance each set of 3 batteries with each other.
Useful? waste of money?
My 6 batteries would mostly go from 100% down to 30% and then back to 100% perhaps 5-6 times a year as this is a "whole-house UPS" that I manually switch on and off when it is needed.

Thanks

Your center string is probably running 5-10% under the outer strings, thus the outer strings MAY wear a little faster than the center string.

Since this is for backup purposes, you should be charging/floating at the 13.6-13.8V float voltage typical for AGM batteries. After 48 hours of floating, you should see all 6 of your 12V batteries at nearly identical voltages.

IMHO, your application (standby with infrequent use) sees negligible benefit from balancers and/or bus bars. Basically, you're 98% of the way to a well designed fully functional system. You're investing a lot of time and potentially money into that last 2% which may not be measurable in terms of cycle life or any other metric.

AGM cycle life to full capacity discharge is typically at least 250 cycles, so the limiting factor on life will be calendar or battery defect (cells short, etc.). Most "standby" type AGM or GEL quote a 10 year life, and that's what I would expect with your system.

 

[dborn]

Great, thanks for confirming. I will then stay with what I have and not try to "improve" it further (I tend to do that )

One aspect of my system I did not mention yet, because it had no impact; this is a kit I bought that includes a 12V 1500W inverter (used at 1000W for efficiency and within the limits of the available power) to generate 120Vac that will power in turn a specially modified 24Vdc 720W battery charger to supplement my 24V 7.2kWh battery bank, all this powered by my EV's 66kWh high voltage battery for extended outages (albeit through the 12V charging system as my EV does not allow connecting to the high voltage battery directly). Yes, alot of losses with all those conversions but this is just to survive power outages with minimal inconveniences.
All this to say that it should not be too hard keeping the 24V battery bank above 50% charge if I keep my house power consumption around 700W on average.
I did add a Renogy shunt to the system so I could keep an eye on the power input/output to/from the battery bank.

Thanks again, your site is really great and I am learning a lot from all you folks and perhaps one day, I can even add solar to this system!

edit: corrected 2500W to 1500W inverter.

 

[SamDeleted]

Just my 2c



If you're only using them for back up, I wouldn't worry too much about the balance of the wiring ... How many times a year will these be cycled? 5/10?

At that rate, they will die from old age well before wear and tear gets them


(Edit. Looks like Eggo already covered all this ?)

 

[SamDeleted]

All this to say that it should not be too hard keeping the 24V battery bank above 50% charge if I keep my house power consumption around 700W on average.

50%dod cycles may well be better for AGM battery's over all health, it's certainly best practice, BUT your batteries should be 100%dod rated (most AGM are, check the datasheet)

If this is the case, don't worry at all about sticking above 50%. For standby use like your doing , you should see no discernible difference in lifespan when comparing 50% cycles to 100%

most standby systems, like UPSs safely use 100% of the battery capacity all the way down to 10.5v per battery

 

[dborn]

Thanks for your insights! ?
When I bought the system last year, I was naturally thinking that if I had 7.2kWh of stored power, I could use it all. Only after reading on many forums and websites that I started seeing "don't go below 30% or even 50% or your batteries will self-destruct!" ?

Of course, even "lower cost" AGM batteries cost about $1500 for six. That's a lot of money to waste on ignorance. Good to read some balanced answers that, yes, you can use it all if it's only going to be occasional. I've read all the documentation (perhaps too much?) on every component of my system and I confirm that it does give numbers of the life expectancy based on usage patterns down to 100% DoD.

My plan is to turn on the house inverter when there is a power outage and just use the batteries for about 3 hours. Then, if it lasts longer, I would get out the extra hardware and connect the EV to help out. Once I get down to ~30% charge on the car, I can unplug it, drive out to the nearest DCFC station (that still has power!), quick charge it back to 80%, drive back home and plug it back in to the home system. With this plan, I figure I could last quite a while. In April, we had an ice storm which lasted almost 48hrs but unfortunately I was still missing some key parts to hook up the car into the system, so after about 14h, my 6 batteries were down to 0% (and stayed that way for another 30hrs. Ouch.). From that point on, I hooked up the 1500w inverter to the car and ran power extensions from the garage to the fridge and freezers and the propane fireplace circulation fan to keep us warm. I've done some changes also to help optimize my house power consumption by replacing all my remaining incandescent light bulbs with some LED ones.

 

[SamDeleted]

Only after reading on many forums and websites that I started seeing "don't go below 30% or even 50% or your batteries will self-destruct!" ?

Of course, even "lower cost" AGM batteries cost about $1500 for six. That's a lot of money to waste on ignorance. Good to read some balanced answers that, yes, you can use it all if it's only going to be occasional. I've read all the documentation (perhaps too much?) on every component of my system and I confirm that it does give numbers of the life expectancy based on usage patterns down to 100% DoD.

Here's a bit more reading on the subject if you are interested. The maths is eye opening to say the least ?

AGM battery Depth of Discharge myth busted

I have been wondering for some time if the "don't discharge your batteries beyond 50%" rule really applies to AGM batteries. AGM batteries claim to be deep discharge, and 50% discharge doesn't seem very deep to me! So I did a little research...

tab-rv.vanillacommunity.com

My plan is to turn on the house inverter when there is a power outage and just use the batteries for about 3 hours. Then, if it lasts longer, I would get out the extra hardware and connect the EV to help out. Once I get down to ~30% charge on the car, I can unplug it, drive out to the nearest DCFC station (that still has power!), quick charge it back to 80%, drive back home and plug it back in to the home system.

Sounds like a good plan, Nice to have an EV for back up! We have a nissan leaf gen2 , it can do power out but no technology is available to take advantage of that in the UK as of yet

Looks like you've got a good few days back up power sorted, it might be worth picking up an LPG(propane) generator if you fear longer outages

 

[dborn]

Here's a bit more reading on the subject if you are interested. The maths is eye opening to say the least ?

AGM battery Depth of Discharge myth busted

I have been wondering for some time if the "don't discharge your batteries beyond 50%" rule really applies to AGM batteries. AGM batteries claim to be deep discharge, and 50% discharge doesn't seem very deep to me! So I did a little research...

tab-rv.vanillacommunity.com

Sounds like a good plan, Nice to have an EV for back up! We have a nissan leaf gen2 , it can do power out but no technology is available to take advantage of that in the UK as of yet

Looks like you've got a good few days back up power sorted, it might be worth picking up an LPG(propane) generator if you fear longer outages

For me, it's a Chevy Bolt EV and I'm in Canada. The company I bought the kit from, has something for the Leaf but I understand that for 240V/50Hz, these kits would probably not be directly usable. I already had an LPG fireplace in the house, with the large tank so I can use that for heat but I'm not adding anymore fuel/gaz burning devices. Only electric from now on

 

[dborn]

Here's a bit more reading on the subject if you are interested. The maths is eye opening to say the least ?

AGM battery Depth of Discharge myth busted

I have been wondering for some time if the "don't discharge your batteries beyond 50%" rule really applies to AGM batteries. AGM batteries claim to be deep discharge, and 50% discharge doesn't seem very deep to me! So I did a little research...

tab-rv.vanillacommunity.com

Sounds like a good plan, Nice to have an EV for back up! We have a nissan leaf gen2 , it can do power out but no technology is available to take advantage of that in the UK as of yet

Looks like you've got a good few days back up power sorted, it might be worth picking up an LPG(propane) generator if you fear longer outages

Based on my battery specs (WindyNation), there wouldn't be any real capacity loss between 100% and 50% DoD. ?
Only going to 30% DoD would yield a higher lifetime capacity down to 60%:

100Ah, 100% DoD, 250 cycles = 25000Ah
100Ah, 50% DoD, 500 cycles = 25000Ah
100Ah, 30% DoD, 1300 cycles = 39000Ah

And frankly even at 60% capacity left, these batteries would still be usable for my application. So it looks like the 10 year lifetime is probably the limiting factor in the end. And I probably won't have much more than 50 cycles in that timeframe.

 

[SamDeleted]

Based on my battery specs (WindyNation), there wouldn't be any real capacity loss between 100% and 50% DoD. ?
Only going to 30% DoD would yield a higher lifetime capacity down to 60%:

100Ah, 100% DoD, 250 cycles = 25000Ah
100Ah, 50% DoD, 500 cycles = 25000Ah
100Ah, 30% DoD, 1300 cycles = 39000Ah

And frankly even at 60% capacity left, these batteries would still be usable for my application. So it looks like the 10 year lifetime is probably the limiting factor in the end. And I probably won't have much more than 50 cycles in that timeframe.

Amazing that isn't it? No discernible difference between 50% and 100% dod cycles (at least on paper ?)

You'll probably only see that 10yrs mark with the correct charge rate and profile, so worth double checking

14.5v absorbion / 13.5v float , these are generally perfect for AGM (temperature adjusted voltage is a real bonus too)

Charge rate is simple, most AGM prefer a c0.15

Example:
1x 12v 100ah battery
100ah x c0.15 = 15amp charger (12v 180w)


Your system:
6 x 100Ah 12v (2s3p)
24v 300ah total
300ah x c0.15 = 45amp charger (24v 1080w)






(Sorry if I am teaching you to suck eggs ?)

 

[SamDeleted]

Charge rate is simple, most AGM prefer a c0.15

Anywhere between c0.10 & c0.25 is usually ok. Of course , it goes without saying, check battery data sheet !

 

[dborn]

Interesting, I was told to set the charging rate to 80A (c0.27) the inverter/charger's max, which seems to be marginally too high. I could dial it down a bit for longevity's sake.
Of course, the only advantage to recharging quickly is to be ready for another outage shortly after the first one ended.
I'll check my data sheet for confirmation though.

Thanks guys,

edit: data sheet says max charge current is 30A for a single battery. Attached chart shows that at 25°C, charging at rate 0.1C would not cause any capacity loss. Am I reading this chart right? I could dial down the charging rate to 30A for my 2s3p batteries.

 

[sunshine_eggo]

That chart in no way relates to charging.

That chart is showing you how much DISCHARGE capacity you get from a fully charged battery discharging at different rates and different temperatures based on a C10 capacity.

Note that at 0.1C and 25°C, you get 100%. - that's your C10 rated capacity.

You get more capacity when discharging at higher temperatures and less at lower temperatures.

Charge at at least 0.1C and no more than maximum for the batteries. If you plan to charge at the battery maximum and multiple batteries are in parallel, make sure current sharing is relatively even with a clamp DC ammeter and that no single battery is over the single battery rating.

Given the standby operation, it really just depends on how quickly you need to be ready for another outage, and that depends on how things have typically gone for you.

 

[dborn]

Yeah, I figured I might not be reading that last chart correctly. There is this one but I have a hard time making some sense out of it ?
Too many lines and too many scales! I do have a clamp ammeter but it's only for AC and besides, my 4/0 cables are too short and too close to the batteries to fit a clamp around them. I think I could lower my charging power around 40A (0.13C) that would allow to recharge from 0% to 100% (300Ah) in around 7.5h as usually when outages are fixed, they don't return for a long while (months?) and having my EV help out, should allow me to not go down to 0% (unless the outage is country-wide and I have nowhere to go charge my car). If that's the case, I better go chop some wood! ?

 

[sunshine_eggo]

That chart is poorly done and it's specific for 25°C.

Ignore the purple/dashed lines. I've added some labels to hopefully clarify:


40A sounds great to me.

 

[dborn]

That chart is poorly done and it's specific for 25°C.

Ignore the purple/dashed lines. I've added some labels to hopefully clarify:

View attachment 180810
40A sounds great to me.

Perfect! thanks for your help, it's appreciated.