A simple battery question

So, if you have a 24v battery bank and a ground wire going to a ground bus and a hot wire going to a charge/discharge bus; do the two wires need to be the same resistance? Or is this hair splitting?

I keep ruining the same battery on my simple two battery 24v bank. That battery does have a couple mrbf fuses on it for continual 24vpower to monitoring systems and CO detection and all the other power is routed thru a Class T fuse. Perhaps those fuses need to be moved off the battery as well. Off the Class T fuse, I go directly to a battery switch to turn on and off the main bus.

I am switching to a LiFePo4 24v battery and don’t want to ruin it as was $900.

I can add a picture, but mostly want to know what could be causing the one battery to keep dying.

 

Does the boat have both 12v and 24v systems or is it all 24v?
How is the current 2-battery bank (2 x 12v in series ?) being charged?

 

Do you turn on the battery bank to "both" when the battery that is continually connected has a low charge?

 

In theory yes, you should have the same diameter and lenght of wire going to both positive and negative bus.

Why you are ruining it is a complex problem. Could be charging voltage, could be charging profile, could be discharging profile, etc. When switching to another chemistry it all becomes irrelevant because you need to configure the whole system for lithium anyway. The lithiums will tolerate incomplete charging and deep discharging a lot better then lead chemistries. I presume you have a BMS to take care of the charging part, so you should be OK.

 

Always charging 24 v

 

You can't have any 12 volt draw pulling off the bottom battery. The only way to have any 12 volt is to install a 24V to 12V converter, or to have a separate 12 volt system in addition to the 24 volt with independent charging.

With lithium, you have to be even more careful than with lead about series batteries. Special chargers than measure and manage the charge of individual batteries in series are available, but the issue often doesn't get talked about until you hit four batteries in series.

Here's an example - not a recommendation, but it gives you something to start researching. Redodo 24V (2*12V) Battery Balancer with LED Display https://www.redodopower.com/products/redodo-battery-balancer-24v2-12v-battery-equalizer-with-led-display?currency=USD&variant=41993430892634&utm_source=google&utm_medium=cpc&utm_campaign=Google%20Shopping&stkn=ab611a9cc912&utm_source=google&utm_medium=cpc&utm_campaign=PMax-Feed-only-0822&gad_source=1&gad_campaignid=21275241550&gbraid=0AAAAAovOnQmpgNAUYAmp45ZP4VNyL_nRL&gclid=CjwKCAjwp_LDBhBCEiwAK7FnkvnxQWOUZwMoDJIjLHhGyL18nk0HFYrof5f5KP5uXGg22Kw9dQVBhBoC3rIQAvD_BwE

 

When I used to work on mobility scooters running 2 X 12 V batts, if the sophisticated charger system got out of whack it would often only charge 1 battery, and the weaker one would die, leaving the operator with much less range and speed. New batteries fitted would soon suffer a similar fate if the charger wasn't checked and adjusted. Charging twin batteries needs excellent charging systems, but a single 24 V battery doesn't suffer the same problems. Your lithium batt should come with its own charger designed specifically for it. As said above, a 12 V tap off needs to go through a voltage converter from 24 V or whatever volts there is available. Or run a separate 12 V battery and system also, from a separate alternator / regulator or charger.

 

I've been using 4s Lifepo4 batteries without a BMS for many years. I balance them sometimes by connecting them in parallel. The charger is cheap RC model charger . The power supply comes from a PC. Lifepo4 cells are boring, which is sometimes a plus. The best quality was probably the A123. I use Haddeway 15Ah/4s. I used them mainly as an outdoor power bank, and recently I've been using them to power my refrigerator. Once I even managed to jump-start a 6-cylinder 2.8-liter diesel engine.

 

This setup was two agm gel relabelled East Penn batteries. I was not pulling 12v. I made that mistake before. These were both brand new batteries. The only odd thing was I had the mrbf fuses on the battery that has failed. Those were my 24 hour 24v circuits. And I kept the Victron charger on all winter. So, their charge cycles were never deeply discharged.

The new battery I am switching to is a Redodo 24v 200AH battery.

I am nervous about using the mrbf fusing on it.

 

Amazon.com https://a.co/d/g8Tkj4P

says it has own bms, I will be charging with Victron and solar and Orion or Sterling. My system has the BMV-712.

I kinda wonder if my battery failed because of being kept on float too much. Maybe the battery with the mrbf fuses (that one died) was getting a bit more somehow.

 

Check connections quality .What exact kind of fuses have been used before ?

 

Switch the batteries to see if that changes the outcome. Disconnect things and measure the resistance, voltage or amps in the system. The amount of torque on connector can create different resistances in a DC system. Stack the largest cable on the bottom of the terminal stack.

 

Why battery is marked 25.6 V ?
LiFePO4 Voltage Charts (1 Cell, 12V, 24V, 48V)

 

That is charging voltage; 24 is nominal.

 

So, I spoke with an electrical engineer friend and he said for a 2 battery 24v series bank, I ought to be using an equalizer. There is really no way to know what caused the batteries to get out of whack unless I find something obvious.

Hope this is edifying.