Running out of battery. Can we mix sizes?

The current through each battery when connected in parallel is proportional to it's internal resistance. So the banks will self regulate and charge equally (unless there's a faulty cell).
 
pteron said:
So in absorption phase I am holding the terminal voltage constant and monitoring the current. My 50Ah battery is more charged than my 110Ah battery (for some unexplained reason). The more charged battery has a higher concentration of sulphuric acid in its electrolyte which causes it to have a higher internal resistance, so it takes less current than the lower state of charge battery. Eventually, the current into the 110Ah battery also falls as it charges and the charger switches out of absorption mode and into float.
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So you don't think you can overcharge a battery by keeping it at absorption voltage too long? In that case all these people who have paid £££ for expensive 3/4 stage chargers have been conned.


If you check the figures you will see that, all other things being equal, your 110Ah battery requires about twice as long as your 50Ah batter (I have no idea why) So either one is being held at that charge for twice as long as it should, or the other for half as long.
 
ghostlymoron said:
The current through each battery when connected in parallel is proportional to it's internal resistance. So the banks will self regulate and charge equally (unless there's a faulty cell).
Click to expand...
That is where you are getting confused. During the absorption phase the charger holds the supply voltage constant (not current as you suggest) - so each battery is independent and is not in any way influenced by the presence of the other in parallel. So the required length of the absorption phase of the smaller battery is not changed by the fact that it is being charged in parallel.
 
pteron said:
So in absorption phase I am holding the terminal voltage constant and monitoring the current. My 50Ah battery is more charged than my 110Ah battery (for some unexplained reason). The more charged battery has a higher concentration of sulphuric acid in its electrolyte which causes it to have a higher internal resistance, so it takes less current than the lower state of charge battery. Eventually, the current into the 110Ah battery also falls as it charges and the charger switches out of absorption mode and into float.
Click to expand...

I think you will find that a higher concentration of sulphuric acid will have a lower resistance.

ghostlymoron said:
The current through each battery when connected in parallel is proportional to it's internal resistance. So the banks will self regulate and charge equally (unless there's a faulty cell).
Click to expand...

Current is inversely proportional to resistance.
 
bedouin said:
So you don't think you can overcharge a battery by keeping it at absorption voltage too long? In that case all these people who have paid £££ for expensive 3/4 stage chargers have been conned.
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You are coming close to agreeing with a post of mine somewhat earlier in the thread.
Troubadour explained that most modern batteries use Pb-Ca technology which makes them very tolerant of "overcharging"
 
ghostlymoron said:
Yes it is. Couldn't be bothered to type an extra word on my phone.
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Remember too that what drives the current through the battery is (Applied voltage) - (Electrochemical potential), so that as the battery gets more fully charged both the effective voltage and the resistance will be changing.
 
bedouin said:
So you don't think you can overcharge a battery by keeping it at absorption voltage too long? In that case all these people who have paid £££ for expensive 3/4 stage chargers have been conned.


If you check the figures you will see that, all other things being equal, your 110Ah battery requires about twice as long as your 50Ah batter (I have no idea why) So either one is being held at that charge for twice as long as it should, or the other for half as long.
Click to expand...

The only reason your 110Ah battery would take twice as long to charge as the 50Ah battery is if you only had a fixed current available to charge it. In a parallel connected system, the two batteries will accept charge at a rate proportional to their capacity. The absorption phase is proportional to C but inversely proportional to the available current. The only issue you are going to have is if the charger terminates absorption based on time. A 'clever' charger should be terminating absorption based on the drop off of current.
 
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