Erratic voltage from battery pack to charge controller

sultan · April 2021

Hello I'm working on a electric conversion of an old John Deere Gator, which is powered with three 300 watt solar panels a Chinese charge controller 60 amp

Going to 16 -3.7 volt lithium ion batteries 185 amp hours, giving me pack voltage of around 50 volts

The DC motor which drives the machine was working fine until 2 days ago, no I'm losing voltage and cannot operate the machine,

When I check with my multimeter at the battery pack shows 51 volts

But the minute I turned on the DC inverter or the solar power to charge it suddenly the voltage is dropping to around 12 volts

When I look at the charge controller screen it's trying to detect the voltage and jumping a radically from 30 volts up to 60 volts and since my charge controller will not charge beyond 55 volts I'm not able to charge it

Any ideas what the problem might be?

706jim · April 2021

For the voltage to drop that much in such a short time, I'd suspect the wiring. Bad connection or a break or somewhere in the wire itself.

BB. · April 2021

Can you tell us a bit more about your battery pack? Type of cells/voltage range.

Does it have a BMS (battery management system)? If yes, can you give a link to it or discribe it (does it shutdown the bank if over/under voltage, etc.)?

A good place to start is use a voltmeter to measure the drop across each cell under load/under charge (or group or cells) if you have access... With a series connection and dramatic voltage drop, you should be able to isolate it pretty quickly to a cell/disconnect/bad wiring/bad connection.

Use a "simple load" (filament lamps are good for this)... A DC motor, especially if it there is "momentum" such as fan/large rotating mass... If the DC power is "interrupted", then the DC motor can become a generator (and it can even "reverse" the +/- DC terminal voltage).

The changing battery bus voltage can cause issues with a solar charge controller... Many will turn off below 9 volts, and will reboot >9 volts, and measure the bus voltage--Then decide if the bus voltage is 12/24/48 volts. If your controller reboots and "sees" 12 volts--Then it may default to a 12 volt battery bus.

Always connect Vbatt to the controller first, and disconnect last. If you have solar power and Vbatt is disconnected, the controller can reboot "confused" or even default to the wrong bus voltage--And potentially be damaged.

The fact that your system worked for a while... It sounds like the battery bank is "near dead" charge wise. With solar systems, it is pretty common for the system to work well for days or weeks+ at a time, then fail because of under charging (not enough sun for daily loads) or over discharging (more loads than the solar can keep up with).

-Bill

Photowhit · April 2021

sultan said:

since my charge controller will not charge beyond 55 volts....

What brand charge controller is this?

sultan · April 2021

thanks for the input

currently i dont have a bms, i plan on putting one on,

when i check the voltage across the individual cells "without a load" they average 3.3 to 3.8v, i havent been able to put a load (dc motor) as the power has been intermittent

but i plan on putting it on a jack and checking each individual cell, i agree that it appears that it is undercharged

the charge controller is the powmr

https://www.amazon.com/PowMr-Controller-Backlight-Battery【Software-Version】/dp/B0719PGGBS

currently i have my solar charge controller directly connected to Positive lead of battery , but the negative lead is connected to the B- of the dc controller rather than directly to the battery

my only other thought was if the dc controller capacitor is somehow sending a voltage to the charge controller tellign it not to charge, or giving it an erratic voltage?

as an update, today it started running again, and showed 57v, but im not sure how accurate that number is under load

scratching my head!

mcgivor · April 2021

@sultan

Reading between the lines, being 3.7V nominal it would appear the cells may be LiPo, if so the low cell voltage of 3.3V is nearing the point of permanent damage, any less than 3.2V they become toasted. Furthermore they seem out of balance, were they ballanced before use or just connected in series?

LiPo cells are amongst the most dangerous chemistries, operating haphazardly without a BMS is a recipe for disaster. Should they in fact be LiPo the voltage required to fully charge would be ~67.2V for 16S, most likely rendering the controller linked unstable for the task.

You really need to submit details because simply saying lithium ion batteries is not enough, the specific chemistry is important, for others to provide any useful feedback.

sultan · April 2021

thank you

to be honest, im not sure what the exact chemistry is, but i do know they were used for solar batteries, im using them for a vehicle off label

i do have a picture of them, from what i do know they are 3.7v 185AH i beleive they are lifepo4

i have attached picture

Image: https://us.v-cdn.net/6024911/uploads/editor/5l/hqvmdwtzrrs7.jpg

sultan · April 2021

i will def add a bms

Erratic voltage from battery pack to charge controller

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