Guide
Do you need a battery monitor shunt, or does your BMS already do it?
A shunt battery monitor counts the amp-hours going in and out of your battery. If your lithium battery has a Bluetooth BMS, its board already does the same thing. Here’s how to decide.
What a shunt battery monitor does
A shunt is a precise resistor in the battery’s negative cable. The monitor measures the current through it, counts the amp-hours, and works out the charge. The Victron SmartShunt is a popular one in South Africa. A shunt sees the whole battery as one, and it works with any battery: lead-acid, AGM or lithium.
What your lithium battery’s BMS already does
Every lithium battery has a management board (the BMS) that measures the current to protect the battery. Most of them also count the amp-hours and work out the charge, just like a shunt. On top of that, the board measures every cell’s voltage, which a shunt can’t see.
If your board has Bluetooth (most JK, many JBD and Daly boards do), an app can read all of it. That’s what CellBoss does: the charge, the current, the power, the time left and every cell, with no extra hardware to fit.
| What you get | Shunt monitor | BMS + CellBoss |
|---|---|---|
| Charge, current and power | Yes | Yes |
| Time to full or empty | Yes | Yes |
| Each cell’s voltage | No | Yes |
| Balancing and the board’s alarms | No | Yes |
| Works with lead-acid batteries | Yes | No |
| Extra hardware to fit | Yes | No |
| Precision at very small currents | Usually better | Depends on the board |
When a shunt still makes sense
- Your battery has no Bluetooth BMS, or it’s lead-acid or AGM.
- You run several batteries of different kinds as one bank and want one number for all of them.
- Your system already has a Victron or similar setup that needs a shunt.
- You need the most precise count at small currents, such as a few milliamps over weeks.
When your BMS is enough
- Your lithium battery has a JK, JBD or Daly board with Bluetooth.
- You want to see the cells, not just the total, so a weak cell doesn’t surprise you.
- You’d rather not cut into your battery cables or pay for extra hardware.
Either way, give the battery a full charge now and then. Both a shunt and a BMS reset their count when the battery is full, and their charge percentage drifts without it.
Read next: why a voltage reading can’t tell you the charge.