Battery monitor jumping to 100% or reporting a high SOC with low Volts / the BMS shuts down

pwfarnell
Thu, 04 Jul 2024 16:10:01 +0000
DIY

This topic covers the reported SOC being out of sync with the actual battery voltage or state. It covers BMV battery monitors, SmartShunts, Lynx BMSs and Lynx Shunts and will just be called shunt below. It is also assumed that the shunt is monitoring the domestic / house / cabin battery bank and not engine start / bow thruster / winch / caravan mover battery banks.

The problems may be one or more of the following and is often linked to mobile applications with small solar systems.

1. The SOC jumps to 100% very quickly after charging starts even though there is not enough time for the battery to have recharged.

2. The SOC is reported to be high, but the battery volts are low.

3. The SOC is reported to be high, but the inverter has shut down.

4. The SOC is reported to be high, but the BMS says the SOC is low.

5. The SOC is reported to be high, but the BMS has disconnected the battery.

If you have set your battery capacity incorrectly or left it at the default setting then the SOC will have no possibility of being correct.

This problem is typically caused by the synchronisation settings in the shunt. There are 3 settings that are involved, the Charged Voltage (this is not what the chargers use), the Tail Current and the Charged Detection Time. The problem is usually that the Charged Voltage is too low and sometimes the Charged Detection Time is too short.

When the battery voltage is higher than the Charged Voltage at the same time as the battery current is below the Tail Current continuously for the Charged Detection Time then the shunt resets to 100%. The logic is for a low current and high voltage, the battery must be full.

However, in a lot of mobile installations the solar charging capacity is low compared to the battery bank size. If the Charged Voltage is set at say 13.5V and the tail current at say 4% on a 500Ah battery bank then if the day is not very bright, the solar panels may harvest enough power to lift the battery above 13.6V but the charge current will easily be below 20 Amps, therefore, after the default 3 minutes, the shunt will reset to 100% before the batteries are full. If you have lithium batteries the voltages are higher than for lead acid, hence exceeding 13.5V occurs very easily. The solution is to set the Charged Voltage to 0.2V below the absorption voltage for a 12V system and prorated for higher voltage systems.

A second problem is on a very sunny day with good solar harvest and the battery voltage is at 14.4 Volt and charging with 40 Amps. A cloud passes in front of the sun and the charging current falls off to a very low value, but because the batteries were at 14.4V it takes some time for the surface charge to deplete and the voltage to fall, so you have high voltage and low current. The solution to this is to increase the Charged Detection Time to at least 5 minutes.

Tail Current is expressed as a percentage of the battery capacity. If you have 100Ah a Tail Current of 4% is 4 Amps, if your battery bank is 500Ah then a Tail Current of 4% is 500 x 4/100 = 20 Amps. If the Tail Current is set too high it is easy to get a false reset to 100%, this should be set as low as possible for your batteries. The best procedure is to watch your charger and see how low the value gets before the charger swaps to float and use a value 25% or 50% higher than this. Flooded lead acid will need 2-3%, high Quality AGM can be around 1% and lithium around 4%.

WARNING
When setting the charged voltage close to the absorption voltage it is important that the tail current falls below the setting on the battery monitor before the solar controller switches to float. The easiest way to arrange this is to end absorption based on tail current and to choose a lower tail current to end absorption than used in the battery monitor. For example, with 500Ah of battery, the battery monitor could be set to 2% tail current, which is 10A. The solar controller can then be set to end absorption at say 8A. Note, if you have for example, the Victron Lithium Smart (or NG) batteries where absorption ends after a fixed 2 hours rather than by tail current, this point does not apply, it only applies if you end absorption by tail current.

There is one final possibility with mobile systems where people often shutdown the complete battery system when the van / RV / boat is out of use. At the bottom of the battery settings page is the option “Battery starts synchronised”, if this is selected every time you power up the shunt it will reset the SOC to 100%, if you are in the habit of often powering down the system turn this off and the shunt should remember the SOC.

To summarise

1. Battery Capacity = set this to match your batteries

2. Charged Voltage = absorption voltage – 0.2V for 12V systems (14.2V for 14.4V absorption)

3. Tail Current = set to suit your batteries, starting points 1% AGM, 2.5% FLA, 4%LiFePO4

4. Charged Detection Time = 5 minutes

5. Battery Starts Synchronised = set to “off” or to “Keep SOC” depending on firmware version

6. Set the solar controller to end absorption at a lower tail current than the one set in step 3.

There is of course another reason not to do with the battery monitor and that is your batteries may well have a problem. For lead acid batteries the capacity of the battery may be substantially reduced through chronic under or over charging, so they charge and discharge much more quickly than expected. Batteries at lower temperatures also show reduced capacity. For lithium batteries, a discharge to a very low voltage would reduce the useable capacity as would charging at high rate at low temperature. Additionally with lithium batteries, if one of the cells is badly out of balance, the battery behaviour may well not match with the observed total battery voltage.

19 posts - 6 participants