I got a Solar-System in my garden because there is no Powergrid. The system contains of a 440Wp Solar Panel, a Bluesolar Chargecontroller, a Smartshunt and two identical Lead-Acid-Batteries connected in parallel. The parallel connection of the two batteries happens at the batterie-pole of the Smartshunt, so both batteries are monitored. In the past few days, I have been dealing with the topic of Peukert.
I calculated the Peukert´s Exponent with the Victron tool. In the configuration of the Smartshunt I put the Peukert´s Exponent and the total capacity of the batterybank (120Ah+120Ah=240Ah). So as I understand the Smartshunt considered the two batteries as once. But this isn´t the real case.
For example: I draw 100 Amps of current. I calculated the discharge time with the Peukert´s Exponent.
With two batteries in parallel the current per battery is in the ideal case half of the total current. So I got a discharge time of ~2,98h.
With one batterie (so like the Smartshunt think it is) the discharge current is 100A. So i got a discharge time of ~1,49h.
So as I expected the discharge time the Smartshunt is showing ist not correct.
Does anyone have the same problem? It would be very handy if I can configure the parallel batteries in the Smartshunt. Or did I miss something? I know the Peukerts´s topic is a approximation to reality.
There is a quite an art to lead acid charge and discharge.
With lead acid the rate at which you draw it out ‘shrinks’ the capacity as i am sure you know. What is the c raring of your batteries to get the 120AH? C10 or C20 - that is the rate you should do the test at to test what their capacity actually is.
So the more gentle the draw the more ‘capacity’ you have. And conversely the heavier the draw the less capacity you have.
That being said mosts systems have variable draws. Hence the approximation remark in the manual.
Assuming both batteries are equal in (age and condition) internal resistance and if they were both fully charged when you performed the test they should share the current but it will never quite be equal because they are never exactly the same. it is best practice to individually connect them to a bus bar for decent current sharing and let them do their own thing (capacity added together in the bmv)
Your batteries should have C ratings (usually C10). Use those. Or slightly less if they aren’t new. Lead acids are the reason the shunt exists so the default 1.25 is usually ok for them.
With 2 batteries in parallel your capacity is 240Ah nominal, but with one, the capacity is only 120Ah.
With2 batteries the 100A discharge is C/2.4, with one battery, the discharge is C/1.2. due to the Pukert exponent, the discharge time of the second case will be less than half of the first case.
With “one battery” I mean the Smartshunt calculated the discharge time thinking there is one battery with a specific Peukert Exponent and a capacity of 240Ah. In the real case there are two batteries with 120Ah each, so 240Ah in total. Because the current splits to the two batteries the discharge time of each battery rises, as i calculated above.
I know the bevaviour of lead acid batteries is a bit special witch make it more interesting for me.
Of course the two batteries aren’t 100% the same, it’s just about the theory.
It’s not a problem like I discribed it above, it’s more like an thought experiment. I haven’t done any testing yet, but I wanted to hear some thoughts on whether my theory is correct.
The peukerts will change as the c rating draw has changed for the bank as a whole. In theory you should actually have a longer discharge time than 2x the battery. So if one was 5 hours for 100ah 2 is not 10hours but closer to 12.
I do not know many lead acid batteries that can do 100A discharge and be ok