Taking accurate voltage readings of batteries?
I have 3 strings of 12 volt batteries in my battery bank. Four batteries in each string. 48 volt battery bank.
Is it even possible to get an accurate voltage reading from any point in the battery bank, given that all batteries are connected? Do you have to disconnect all the batteries to get true voltage readings from each battery?
I am also regularly checking specific gravity on all 72 cells with my hydro-volt hydrometer.
Also, I have a Schneider battery monitor that monitors 2 midpoint voltages (wish it had one more mid-point voltage tap wire so I could monitor the third string without using my volt meter).
While the battery monitor does display each of the two tapped midpoint voltages, can you actually trust that reading to be accurate, given that again, the batteries are all connected and as such, are having their voltage readings affected by the other batteries connected.
Comments
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
As Mcgivor notes, current balance while charging is the important metric. To some extent though, this could be implied indirectly with individual voltage reading by seeing some rising faster in voltage than others. The fast ones end up overcharged, and the slow ones undercharged. Over time, the slow ones get slower, etc.
I've sometimes used individual battery voltages at around the start of absorb as a quick and dirty check between SG readings to see if the SG check should be done sooner than planned.
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
Under the circumstances, I'd be inclined to separate the new ones and go through a full charge/EQ on them alone. If your loads allow, running on just the new ones for a few days might help too.
I got a usable DC clamp meter from amazon for ~$50. @bb had links to some in recent thread(s).
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
https://www.amazon.com/gp/product/B019CY4FB4 ($105)
https://www.amazon.com/gp/product/B07546L9RT ($40)
The "cheap one" may come with a Chinese only manual... So you may have to play around to learn how it works (or ask a few questions here).
-Bill
When building a LFP battery the topography is opposite to how lead acid would be, all cells are first arranged in parallel to achieve the desired capacity then series to to get the required nominal voltage, this means there are no parrallel batteries, just parallel cells. Using multiple unitized pre built batteries in parallel may have advantages that I'm unaware of, other than redundancy perhaps , but having a single BMS looking after the entire bank definitely sounds simpler, to me at least.
Understandably some don't want to build a DIY LFP bank, prefering manufactured solutions for personal reasons, but building your own is not difficult and once operational is definitely much easier to maintain than a LA bank. With a good BMS there is protection against many potential problems which may occur, things a lead acid bank would definitely benifit from but typically do not, which is why there is so much discussion regarding low cell capacity, over changing, over discharge etcetera.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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
The cost really depends on where the cells are sourced, the BMS used and the capacity required. For the bank I constructed using 32 × 100 Ah prysmatic cells, 4P 8S arrangement the cell cost directly from the manufacturer was $2080, shipping $250, BMS $100, second passive BMS for its display $110 and a 3.65V single cell charger $65 for a total of ~ $2600 for a 10 Kwh bank or $260 perror Kwh nominal . This is about double the cost locally for LA 400Ahusing golf cart batteries, but the comparison is skewered because LA only has <50% useful capacity to get somewhat decent cycle life, as opposed to 70% conservative with LFP.
The Discover AES is $7200 for 6.6Kwh or ~$1000 per Kwh nominal, but does have a 10 year warranty, communication ability with certain equipment wrapped in a nice container, should these things be important. The cell cost in the US for reputable cells is about $125 for a 100Ah cell so a DIY bank would be ~$500 per Kwh. https://www.ev-power.eu/CALB-40Ah-400Ah/CALB-CA100FI-Lithium-Cell-LiFePO4-3-2V-100Ah.html In the end it comes down to personal preference and requirements. The Discover AES claims 6.6Kwh usable which suggests it too may be closer to 10Kwh actual, if so the price would be more attractive.
This link has some useful information.
http://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
A security light project might be a place to start
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