New 24v Lithium bank & system cabin, separate 12v genset starter battery

[TJ King]

I am replacing my old 12v Lead acid system with 24v lithium and new MPP LV2424 inverter/cc, but need to keep one of my 12v Lead acid battery to start my generator and/or run a couple 12v items (10 amp pump, etc). Generator draws less than 10A to crank and has its own 10A fuse. I was looking at Will's "2000W 24v off grid" How to, and he recommends the 24 to 12v 40amp converter step down (below). My questions: Do I need a 40a for my limited loads? Cant I use lets say a less expensive 20A? ...This is just to keep the starter battery topped off and occasional 60 seconds of water pumping, is this appropriate to "float charge" my battery? I still have my old pwm controller, so cant i just feed 24v dc into the PWM charge controller and have it provide 12v float charge to my Lead Acid gen set starter battery? Im even considering just putting a small 30w panel on the roof near my new array to provide energy to the old pwm and maintain a completely separate self sustaining 12v system, just to start the generator & pump. Any ideas?

(https://www.amazon.com/dp/B01LYK6G2...a6ade415085fbe081d9edcd496d45b&language=en_US)

 

[snoobler]

A regular 12V converter will kill your battery. A battery @ 12V is below 50% charged and it will eat itself.

Better to get one at 13.8V float voltage:

Amazon.com: KNACRO 13.8V Buck Voltage Vonverters 24V (18v-36v) converts Step Down to 13.8V Step Down car Power inverters Input DC 18V-36V Output 13.8V 30A Waterproof Power adapters (30A) : Everything Else

Amazon.com: KNACRO 13.8V Buck Voltage Vonverters 24V (18v-36v) converts Step Down to 13.8V Step Down car Power inverters Input DC 18V-36V Output 13.8V 30A Waterproof Power adapters (30A) : Everything Else

www.amazon.com

They have 10, 20, 30 and 40A.

A PWM charge controller can't limit current, and it shorts the source to the battery, so your results might very "unfavorable" since your 24V source can supply LOTS of amps.

 

[TJ King]

A regular 12V converter will kill your battery. A battery @ 12V is below 50% charged and it will eat itself.

Better to get one at 13.8V float voltage:

Amazon.com: KNACRO 13.8V Buck Voltage Vonverters 24V (18v-36v) converts Step Down to 13.8V Step Down car Power inverters Input DC 18V-36V Output 13.8V 30A Waterproof Power adapters (30A) : Everything Else

Amazon.com: KNACRO 13.8V Buck Voltage Vonverters 24V (18v-36v) converts Step Down to 13.8V Step Down car Power inverters Input DC 18V-36V Output 13.8V 30A Waterproof Power adapters (30A) : Everything Else

www.amazon.com

They have 10, 20, 30 and 40A.

A PWM charge controller can't limit current, and it shorts the source to the battery, so your results might very "unfavorable" since your 24V source can supply LOTS of amps.

So at 13.8v it will float charge the lead acid but will never reach boost charge levels, right? But I thought a typical float charging charger hits a certain peak then drops volts to 13.8 or so but also drops amps down to a gentle "trickle", or maybe I'm assuming. For my application, I'm guessing the most gentle solution would be the 13.8v 2amp model, yes? Or am I imagining the concept incorrectly that a 40A 13.8 charge is pushing 40a onto the battery all the time?

 

[snoobler]

So at 13.8v it will float charge the lead acid but will never reach boost charge levels, right? But I thought a typical float charging charger hits a certain peak then drops volts to 13.8 or so but also drops amps down to a gentle "trickle", or maybe I'm assuming. For my application, I'm guessing the most gentle solution would be the 13.8v 2amp model, yes? Or am I imagining the concept incorrectly that a 40A 13.8 charge is pushing 40a onto the battery all the time?

It will take a longer time to charge, but it will eventually reach full charge. Standby applications like a UPS are simply charged in this manner to 13.8V and held there. Many batteries publish two charge voltages - Standby and Cyclic. Standby is used so infrequently, 13.8V is sufficient to fully charge them, but with cyclic, you need the higher voltage to ensure they get fully charged each cycle.

Batteries will only accept as much current as they can and maintain the set voltage. Once charged, it will likely barely accept a Watt or two.

 

[TJ King]

It will take a longer time to charge, but it will eventually reach full charge. Standby applications like a UPS are simply charged in this manner to 13.8V and held there. Many batteries publish two charge voltages - Standby and Cyclic. Standby is used so infrequently, 13.8V is sufficient to fully charge them, but with cyclic, you need the higher voltage to ensure they get fully charged each cycle.

Batteries will only accept as much current as they can and maintain the set voltage. Once charged, it will likely barely accept a Watt or two.

Thank you for getting me on the right path

 

[LVLAaron]

You could use almost any cheap CCCV charger on amazon.... just set the output to 13.8 to keep your 12v battery charged. Don't overthink it.

 

[Bubba1]

Victron has a 9amp battery to battery charger for $65 . That's the 12v to 12v. The 24 to 12 is probably a little more $. Look on Amazon. A 50w panel would probably give you 10 to 15 amps a day@ 12v.

 

[TJ King]

You could use almost any cheap CCCV charger on amazon.... just set the output to 13.8 to keep your 12v battery charged. Don't overthink it.

I got a 24 to 13.8v step down buck charger, but had a problem when my 24v bank ran down & hit BMS cutoff the buck charger back fed 13.8v back to my lv2424 inverter charger causing a fault and hopefully no damage. Not sure if a diode or isolator would fix that and if so which one. Over and underthinking this one