I have been looking at replacing a weak 5 AHr SLA battery for a while.
In looking at options, I discovered after paying shipping costs, a new LiFePo4 batteries were getting quite competitive in cost. Considering that they are far lighter, and last a lot longer (at least double or triple the recharge cycles if charged and discharged with proper care), I decided to purchase a small LiFePo4 battery for a lightweight backup for several possible functions.
I already have two 50 Ahr LiFePo4 batteries,and I am very happy with them. .\
I found a 6 Ahr LiFePo4 battery with a built-in battery maintenance board (BMS) for about $30 on E-bay.
A BMS can do several things: It can cut off the discharge if the battery voltage goes too low. It can cut off charging current if the voltage or temperature goes too high. It can regulate charging currents to the various series connected cells to provide balanced cell charging, and there are other possible functions depending upon design. .
When the new battery arrived, and even before reading the 4 page instruction manual ( You all read the manuals don't you?), I hooked a small 12v LED lamp to the battery.... and nothing happened.
OK ,it must need to be charged...
Hooking the new battery to a metered variable voltage supply, the voltmeter showed NOTHING!
Normally the built-in voltmeter on the power supply shows
the voltage of any battery hooked to the power supply, if is turned on.
Nothing is not what anybody expects to see, even on a seriously discharge battery...there is always 'something' to show on a DVM...so I tried the DVM, and measured 3.6 volts across four cells in series, but if I hooked an analog meter to the some test points at the same time, the DVM measured 0.2 to 0.3 volts and dropped slowly. there was certainly no usable power there.
This was more than a bit concerning, so I sent a note to the E-bay seller explaining that I could not get the battery to accept a charge. .
The next day, I received a nice note back telling me I needed to use a charger designed for a LiFePo4 battery.
I knew that; for long life the correct charger was desirable, but I also know a LiFePo4 battery is still just a battery, and if a higher potential is applied, current should flow to charge the battery, if the battery is capable of accepting a charge current.
I have a proper battery charger for that battery chemistry, so I dug it out and hooked it all up with a DVM and an in-line ammeter.....Nothing! NO current wold flow from a 14 volt charger to a battery pack that measured less than 4 volts with a DVM, and 'nothing' with an analog meter set to the 2.5 volt range.
Reporting that to the supplier, I received a prompt note back, to "Please send pictures of my test setup".
That meant clearing a space on the bench, so that somebody looking at the photos could differentiate which wire was hooked to what, instead of seeing the normal jumbled pile of 'stuff' I often work on top of. ( Actually the bench cleaning was a good thing...I found some items I did not know were even missing... I can hear Mihai laughing now).
The end result of my testing, as reported to the seller, was that either one of the battery pack connections inside were broken, or the BMS was defective.
I received a nice note back thanking me for the good pictures, and that a replacement battery would be shipped soon. I was told I did not need to return the defective battery.
So...Today I sawed the battery case apart and wired around the suspect BMS. The actual 4 cells of the battery pack are just fine, and accept a charge normally.
In the mean time, I have a usable battery, and will be looking for a suitable replacement 4-cell BMS card.