# Mid point Voltage Taps and Battery Monitor

Registered Users Posts: 209 ✭✭
Tonight, I was giving my 3 string battery bank, its monthly charge up and 1 hour EQ, to mix up electrolyte and noticed .15V difference in mid point voltages from one string to the next. Times Two = .3V (48V nominal). I was concerned that the strings could be straying so soon.....

However upon closer examination of each of the 12 batteries, while they were in absorb, I took individual voltage readings of each battery and found the 12V battery, where one of the mid point taps is attached, was actually reading, over all, a little lower than the other battery, in one of the other strings mid point tap battery.

So if that does not sound too confusing,.......... the mid point taps happen to be connected to two batteries of different resistances and voltages, thus deviating mid point voltage readings on my battery monitor.

So my question is, what good is a mid point tap? Voltage is just an indicator anyway (kind of useless) second to actual current flow measure by a DC clamp on meter and Specific Gravity readings measured with Hydrometer.

I guess in a perfect world with all batteries perfectly manufactured, with each cell having exact resistance and all batteries perfectly balanced, mid point voltage taps could be useful to spot an imbalance early, but since this is not the case I find myself questioning the designed function of mid point voltage taps altogether???

Maybe the .3V (48 nominal) difference in Voltage from one string to the next, is the mid point voltage tap proving its validity, however when I take the average voltage, of each of the four 12 volt batteries in each of the 3 strings, they are pretty much bang on equal. Some batteries are a little higher and some a little lower but it all averages out.

So, I may separate the strings from time to time to charge/ EQ each string on its own, but, if all my S.G. reading are bang on, I really don't care what voltage deviation I'm getting during charge or loading since at the end of the day S.G.being equal is all that matters,.....Correct???

Anyone care to comment or clarify:)

• Solar Expert Posts: 9,583 ✭✭✭✭✭
Expecting mass produced FLA batteries to be "Identical" is unrealistic.

Generally, adding a bit of extra Absorb or Float time, will "slightly overcharge" some cells, while fully charging the others.   The monthly EQ can also do that, but gasses the batteries more because of it's higher voltage.

You have to ride the razor between under and over charging.  Under charge leads to sulfation, over charge to + plate corrosion.  Nearly all cells fail from undercharge, and over charging in the solar power field, is rare.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 6,863 ✭✭✭✭✭✭
Those are words of wisdom Mike
"we go where power lines don't" Sierra Nevada mountain area
htps://offgridsolar1.com/
E-mail offgridsolar@sti.net

• Solar Expert Posts: 9,583 ✭✭✭✭✭
Dang, I must be getting old !

Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 209 ✭✭
edited January 2020 #5
Does any of the experienced people here even use mid-point voltage taps?  I guess if your battery monitor comes with it, why not hook it up, however, they seem kinda useless?
Any last thoughts in this post?
• Registered Users Posts: 4,496 ✭✭✭✭✭
I do spot check mid-string voltages occasionally, but it's regular SG checks I really rely on to monitor battery health.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Solar Expert Posts: 1,386 ✭✭✭✭
You could add a series battery balancer (eg, HA02).    It will eliminate the voltage difference, but I don't think anyone has data on how much this helps battery life.

I am available for custom hardware/firmware development

• Registered Users Posts: 209 ✭✭
I have been considering battery balancing for a while and have a post relating to this topic here:
That being said, I'm going to leave the mid-point voltage tap post and return to the battery balancing post.
Thanks for the suggestion jonr. My name is John R as well. Thats funny
jonr if you care to review the battery balancer post linked above I'll be continuing the discussion there. I've a challenge on my hands attempting to automatically and safely balance 3 strings of four-12 volt batteries. 12 batteries in total.
It is possible without additional equip, however I am just trying to make the process more stream line for a customer.