Too Much Current Into Discharged Battery?

[rardiH36]

Just wondering about something and nothing readily comes up on a Google search.

I have on my boat a relatively small but adequate for the purpose emergency starting battery. I maintain it's readiness state with a small solar panel. If my group 27 main batteries get into trouble, I can disconnect them from the circuit, then switch to the emergency battery to start the engine. Crude solution, but it gives some peace of mind.

Say however that I have depleted this emergency battery to maybe 12.2v = 30-40% state of charge. My main batteries however are fully charged. I then accidently engage the depleted small emergency battery into the circuit.

Will the sudden availability of several hundred cranking amps damage the starting battery?

 

[Don S/V ILLusion]

From your description, I don't see how you could combine them "accidentally" but regardless, the answer is no.

 

[Johnb]

Agree with Don, the small difference in potential, the speed with which the voltage in the discharged battery rises, the impedance of the connections and the thermal mass of the battery all combine to make this a non issue.

 

[Maine Sail]

Will the sudden availability of several hundred cranking amps damage the starting battery?

No it won't.

 

[Bill Roosa]

The V in the equation V+IR is the differance in voltage not to ground (0 volts) so 12.2 to 12.6 in the two batteries only has a 0.4 volt differance (and internal battery resistance on both batteries..........
No it will not.
BTW we do this all the time on boats with the three way switch. Drain the house batteries over night then switch from house to start and go through "both" in route. Or just combine all the batteries......nothing exciting happens

 

[Ralph Johnstone]

Just wondering about something and nothing readily comes up on a Google search.

Why go to Google when you have all of this (SBO) to dive into ? ? ? ?

If my group 27 main batteries get into trouble, I can disconnect them from the circuit, then switch to the emergency battery to start the engine. Crude solution, but it gives some peace of mind.

Nothing crude about that. I think that's the way most smart starting circuits are arranged. The starting battery is treated as sacred and not touched until the day arises when it's the ONLY source of cranking power remaining. Now, having said that, I still start the cold, cold engine every month or so on the starting battery, by itself, to ensure the battery's still in good shape.

 

[Benny17441]

The transfer of current will not exceed the acceptance rate of the discharged battery. Just the same as if you would have plugged in the battery charger. If I gather correctly you are just carrying a spare battery in case of emergency. We do that when we head for the Bahamas.

 

[rardiH36]

Thanks for all of the responses. Now I know!

Why go to Google when you have all of this (SBO) to dive into ? ? ? ?

Agreed! SBO and the search function is a fantastic resourse. I do most often search SBO rather than google. But in this instance, I couldn't recall seeing (or maybe just not noticing) this topic during my seven years with SBO. And it seemed more a generic led-acid battery question rather than boat specific.

Nothing crude about that. I think that's the way most smart starting circuits are arranged. The starting battery is treated as sacred and not touched until the day arises when it's the ONLY source of cranking power remaining. Now, having said that, I still start the cold, cold engine every month or so on the starting battery, by itself, to ensure the battery's still in good shape.

Although I had previously wondered about connecting a depleted battery (in parallel) to a fully charged one before, the need to know became "live" last week. As Ralph Johnstone also does, I periodically test the back-up battery. After switching off the main group 27 batteries, I engaged my emergency battery into the 12V circuit to do an engine start. (It's a real small thing, just a Duracell U1 case "heavy duty" 350 cca home tractor starting battery. Starts my old Yanmar though fine without hesitation.) But then afterwards, I neglected to switch it out again from the 12v circuit. Nor did I throw the switch to connect it back to the small solar panel. And I did not re-engage the group 27 batteries. Returning to the boat after several days of small drain demand, including my hand-held backup VHF radio and boat tablet that were plugged into the re-charging station, I found the U1 battery at 12.2 volts. After admonishing myself (particularly since I always turn my primary batteries' A/B/Both switch to off when I am away from my boat), I just switched it off from the 12v system, got the solar charger back on line, and went about my normal routine of using the main house batteries...

... But I did wonder if the small much smaller and partially drained back-up battery would have been "overwhelmed" had I re-engaged the house batteries before disconnecting it.

 

[Ralph Johnstone]

Ouch ..................

But then afterwards, I neglected to switch it out again from the 12v circuit. Nor did I throw the switch to connect it back to the small solar panel. And I did not re-engage the group 27 batteries.

......................... three strikes

It could be argued that such errors can be overcome by installing all manner of electronic safeties but that would only add to the complexity of the system. I prefer to keep the circuitry as simple as possible in those areas (engine starting) and rely on repetition and memory.

Not much different than heading into some of our northern narrows and forgetting to check the timing on the tide and current tables. You'll only do it once because:

a) You won't be alive to make the same mistake again.

OR ...............

b) You'll be so terrified, you'll never make the same mistake again.

Experience really is the best teacher .