Battery Multiple-choice Question

[Rick D]

As a follow up to my long winded marathon battery failure post, I am in the process of planning my re-wire. So, here is my question: I decided about six years ago that it would be a good idea to keep my big house bank from deteriorating by adding a second bank of a small group 24 deep cycle battery. I keep the boat plugged in at the dock and run a refrigerator and freezer 24/7. I mad a judgement that this constant load and charge couldn't be good for the big house bank and so for the purpose of running the refrig at the dock, I would use the small "sacrificial" group 24. I combined the banks when motoring and cruising.
After some years of that, I was advised by a friend who ran big UPS systems for industrial purposes that that wasn't going to help my big bank. He said industry runs big systems and the worst thing you can do is simply leave batteries idle. Thereafter, I just left everything combined as one big bank. However, I recently ran into a few boaters who did the same as I with the same rationale. So, now I am in a quandary about removing that small battery or leaving it and isolating the big bank while in the slip.
So, do you think:
A) your original thought to use a small battery to save the big bank was a brilliant piece of deductive reasoning and you should continue to use it, or
B) I have never heard such an inane, flawed piece of amateur logic and you should remove that disgusting apparatus, or
C) I don't know, but you should listen to your experienced friend, or
D) this is what the real facts are (insert facts).

 

[dlochner]

E) Use an automatic combining relay (ACR) to combine the house and start batteries. Leave refrigerator connected to house battery.

 

[jssailem]

Dave, My read of Rick's question is that he may have 3 sets of batteries: his big bank that he is saving, his refirgerator dock bank, and maybe a starter motor bank.
But that said, Dave's answer is the current model of thought on battery management. Hook the House system together into a single bank. Have a small (i.e. group 24 what ever is needed by your engine to start) battery as an emergency starter battery. Use your house batteries for your everyday use. Use the ACR to provide charge to both battery groups. Have switches to isolate the banks should you need to do so (i.e. the house bank develops a short and dies - do not connect a good starter battery to a dead bank).
Rick... suggest reading the volumes of supporting info provided by MaineSail and Stu Jackson on this forum.

 

[Davidasailor26]

I pretty much agree with dlochner, although I think that's a subset of choice (D). Use a 24 as a start bank if you want, with an ACR to keep it charged. A sufficiently smart charger connected to the house bank will handle the refrigerator load without hurting the bank. Mixing different size/types of batteries within a bank is not advisable.

 

[Rick D]

E) Use an automatic combining relay (ACR) to combine the house and start batteries. Leave refrigerator connected to house battery.

That's exactly what I'm doing. Purchased the ACR from this site. However, what about the theory of using just a small battery to run the refrig at the dock to "save" the big house bank?

 

[Rick D]

I pretty much agree with dlochner, although I think that's a subset of choice (D). Use a 24 as a start bank if you want, with an ACR to keep it charged. A sufficiently smart charger connected to the house bank will handle the refrigerator load without hurting the bank. Mixing different size/types of batteries within a bank is not advisable.

That's how I'm leaning; I have a Heart 2500 inverter/charger that should still be pretty smart. It sounds like you agree with my friend that I'm not going to hurt the big bank by continuously having a refrigerator draw on it at the slip.

 

[dlochner]

However, what about the theory of using just a small battery to run the refrig at the dock to "save" the big house bank?

The simplified answer to this question is using the start battery to run refrigeration will do proportionally more harm to the small battery than to the large battery. Unless the charger is large enough to serve as a power source for the refrigerator.

Deep discharges with inadequate recharging reduces battery life. A group 24 battery has about 75 ah capacity, drawing it down by 25 ah takes it to a 75% SOC. The same 25 ah draw on a 460 ah bank takes the bank down to about 94% SOC. The big bank barely notices the decrease, the little bank is getting beat up. Now this is the simplified version of the story. Old Dr. Peukert gets involved in this and makes the calculations a little different, but the effect is the same.

I have Grp 24 start battery and 460 ah house bank (4 GCs) when my refrigeration kicks in the battery voltage starts to drop, the charger senses the drop and starts supplying enough power to run the refrigerator. The charger is a 50 amp ProNautic that has ample power to run the refrigerator that draws ~30 amps. If the charger were smaller than 30 amps the charger would supply whatever it could and the battery would supply the rest. Once the refrigerator shut off, the charger would continue running until the battery was charged.

Refrigerator cycling times may also have a role. High load short cycles may not allow sufficient time between cycles to return the battery to a full SOC. A charger capable of providing all the power to run the refrigerator results in very little drain on the battery, whereas a smaller charger will rely on the battery to supplement the power supplied by the charger. If the charger is unable to return the battery to 100% SOC before the next cooling cycle begins, a downward spiral begins. And you end up with warm beer come the next Friday evening.

 

[Davidasailor26]

Our fridge draws about 5 amps when running, and it runs about 50% of the time. Our 20A charger has no problem keeping up with the 5A load. The only thing you don't want is for the fridge load to trick the charger into thinking that the battery is low, at which time it will go into bulk charge mode at over 14 volts, rather than floating at 13.something like it should.

 

[jeepbluetj]

Big enough charger will power the fridge just fine. Combining the G24 into your big bank could have less than optimal results (not matched batteries) - Also, depending on your fridge controller, some fridges run on 110 too - and that's a great way to not worry about the problem.

 

[Maine Sail]

If the fridge is on at the dock the battery charger is essentially running the fridge, not the battery. So long as the charger can remain in float, during fridge start ups, then the battery is unaffected by the fridge because the charger is powering it.. Float is float to the battery bank. If your charger gets bumped out of float due to the fridge kicking in create a "float only" profile or select a setting that puts it at a continuous float voltage,

 

[Rick D]

Thanks guys! The charger is 120 amps as I recall, so it's never pushed the batteries out of float even though I have separate frig & freezer compressors. The start battery wasn't involved; it was a separate group 24 deep cycle which I had on at the dock. That's a relief. Good to know I am not harming the big house bank. So it looks like it's B that carries the day!

 

[Ron20324]

.... Our 20A charger has no problem keeping up with the 5A load. The only thing you don't want is for the fridge load to trick the charger into thinking that the battery is low, at which time it will go into bulk charge mode at over 14 volts, rather than floating at 13.something like it should.

I set my Xantrex charger to the 13.2 setting to provide power for the fridge at the slip. That way I don't need to worry about the bulk/float voltages.