Lithium Battery Charge Level Monitoring

[poindexter]

Hey Forum,

I am doing some work on setting my electrical system. I have x2 lithium leisure batteries and have been researching how they work so I can monitor charge levels effectively.

I came across the below graphic which shows how a lead battery discharges vs how a lithium battery discharges.

Surely this is going to make it very difficult to monitor how much charge is in the batteries? I know its possible to though as my escooter has lithium batteries in it.

Any suggestions how one would proceed with monitoring charge levels on x2 lithium leisure batteries? Any recommended products?

 

[Lenny HB]

Victron Smart Shunt or Victron BMV712 battery monitors will give you accurate monitoring of state of charge. They work by counting the amps in and out.

 

[Two on Tour]

As Lenny says, using a shunt is pretty well the only way of getting an accurate voltage and SOC reading of your lithium batteries.
There as cheaper shunts with readout out there but if you want accuracy, then a Victron shunt is the way to go.

 

[oldiesontour]

As already said a BATTERY MONITOR is what you need but has to read amps as well as volts, this works by measuring current in and out of battery,calculates what's in battery and gives ah and percentage left in battery,any shunt wired meter will give a more accurate reading than a coulomb type meter,shunt type can be bought with a Bluetooth output to monitor on your phone as well as screen reading,or without Bluetooth There are very differing prices and one particular Chinese fairly cheap one gets lots of good reviews (search battery monitor reviews) at the top end as always victron come highly recommended but carry a bigger price tag

 

[Cush]

If they are getting regular use, and you don’t have sub 1 or 1.5 amp drains would the bms not give an “accurate enough” indications of state of charge? I understand that there are apps that connect to multiple bms’ if your batteries have the jbd bms. Or do you need a readout in the van?

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[Guigsy]

You are right. Monitoring lithium batteries based on voltage is nearly impossible. Instead we use shunts. They just measure current. Add up the current on and out over time and the smart shunt can pretty actually estimate the state of charge. Most lithium battery BMSs have a shunt built in.

 

[Pausim]

You are right. Monitoring lithium batteries based on voltage is nearly impossible. Instead we use shunts. They just measure current. Add up the current on and out over time and the smart shunt can pretty actually estimate the state of charge. Most lithium battery BMSs have a shunt built in.

Not sure that most have built in monitoring. My current one doesn’t and the pair I intend to buy for my new van don’t. The cost of the ones I want are £124 more each with bluetooth monitoring and 2x£124 is lot more than a Victron BMV.

 

[LithiumConvert]

I came across the below graphic which shows how a lead battery discharges vs how a lithium battery discharges.

That graphic is clearly wrong. Showing the same voltage froodom 100% to 25%.

Look up a good chart and you can get some useable figures. (Still better to buy an Energy Meter, though)

 

[Pausim]

This is the graph for my Relion battery. The line from 95% to 20% is so flat it is difficult to get a helpful state of charge from the voltage. Then add in the rate of discharge making a bigger difference than the state of charge and it renders voltage a very poor indicator.

 

[headlight]

Voltage is a very inaccurate way to measure SOC of lithium due to their very flat discharge curve.
As others have said really the only accurate way is a shunt.
Shunts can be had for as little as £50 and are far more accurate than the BMS.
I have Fogstar LifePo4 which has a BMS and app.
However the BMS cannot detect low drains, in my case it doesn't register 1.5 amps and below.
Therefore I could drain my battery by up to 36 amps a day without any of that amp usage showing ,on my BMS/app.
I have a Victron shunt that does detect that low drain so gives an accurate SOC.

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[autorouter]

To keep track of the state of charge (SOC), the alternative to voltage measurement is to count the amps in and out of the battery. For example, a 1A current for 12 minutes (1/5 of an hour) means a charge of 1/5 amp-hours, ie 0.2Ah. To do a measurement like this requires a shunt to measure amps, and a small microprocessor to measure time and add all the Ah bits together and keep track of the total amount.

This method is much better than voltage measurement, and can show the SOC even while the battery is under load or being charged, when voltage measurement is hopeless. However it's not a magic fit and forget solution. It needs to be told the capacity (Ah) of the battery, and also to be initially told when it is full. Then it can count the charge in and out, and calculate the SOC percentage. This initial setting of the 100% level is usually called 'synchronisation'. Often it is automatic, so it takes maybe a couple of fill/empty cycles to settle down to an accurate readout. Sometimes there is a button to 'Sync Now' so you can manually sync if you know for example that it is already fully charged when you first install it.

The shunt is usually installed immediately next to the battery negative terminal. Because that location is a bit inconvenient for easy reading, most shunts have a separate display panel that can be mounted in a more convenient place. Some instead just have a smartphone app and send data via bluetooth, with no display panel and no need to route a wire for the display.

 

[LithiumConvert]

I find this chart is a good reference with my Sterling Power batteries. It matches the Energy Meter quite well.

 

[Lampwick]

Can I attach a victron shunt to the car battery. (none smart alternator) I have one for the leisure batteries already which now gives me accurate information.

 

[Ray-Vic]

Can I attach a victron shunt to the car battery. (none smart alternator) I have one for the leisure batteries already which now gives me accurate information.

If you use the Victron Smart Shunt on your leisure batteries you can add your cab battery to the aux terminal to monitor it. As your cab battery is almost certainly a lead acid battery, voltage will give you a reasonable measure of SOC (but not battery condition).

If you don't have this capacity or it's a bit of a faff running a cable between your cab battery and shunt, you could install one of these which will let you use your Victron bluetooth app to see the cab battery voltage.

 

[tonka]

I recently fitted 2 x Ecoworthy 100amp lithium's, these don't have Bluetooth. On recommendation I have fitted the Victron smart shunt, early days as only done one night away to check. All seems ok, works via app on phone.

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[autorouter]

Can I attach a victron shunt to the car battery. (none smart alternator) I have one for the leisure batteries already which now gives me accurate information.

The problem with using a shunt on the starter battery is that, for meaningful measurement, the shunt must be on the negative terminal and take ALL the starter battery amps in and out. That means it has to be able to measure the starter motor amps, which will be several hundred amps. Very likely exceeding the rating if it's a 500A shunt. You can get bigger shunts, 1000A or more, but then that will have trouble at the other end of the range, measuring the very low amps trickling in and out of the starter battery over days and weeks. Like the alarm, tracker etc. And if it doesn't measure those, there's no real purpose in having a dedicated shunt in the first place.

Overall I think it's best to just monitor the voltage level to keep an eye on the starter battery condition. Either as Ray-Vic describes with a voltage monitoring wire from the existing Victron shunt, or a dedicated meter. One option if you are OK with a smartphone app is a Victron 'SmartSense' module, that literally sticks on the side of the battery, with a couple of thin wires to positive and negative. It sends voltage and temperature data to the Victron app via Bluetooth.