Relay BMS Units: The Good, Bad, & Ugly

Control & Monitoring systems aside, three types of relays could be considered for use in a BMS; Normally Open, Normally Closed, and Bi-stable latching.
AUTHOR’S NOTE: We are excluding Solid-State relays since these would be the exact equivalent to using a MOSFET-based BMS.

When current is applied to the coil of a normally-open relay, the contacts are connected and current can flow from the battery to the loads.  The downside is that any time current is expected to flow, the coil must be powered, which consumes power continuously.  Let’s say a relay draws a mere 10 watts – operating this relay for 1 day consumes 240 Wh, 1 week consumes 1.68 kWh, and for 1 year consumes 87.3 kWh.  This is not something to gloss over – larger relays can consume over 7 times this amount.  The benefit here is that if something goes catastrophically wrong, power to the coil is most likely lost, and a system fails in an “off” state.

Normally closed relays are the polar opposite of a normally open relay. When current is applied, the contact is separated, breaking the circuit to the loads.  During normal use, the issue of consumption by the coil is avoided, however, three new issues have been created.  If a BMS were to trigger protection because the battery cells are over-discharged, then the BMS cannot stop all loads from consuming power from the battery because the relay itself becomes a load.  This would lead to a further decrease of battery charge, and could quickly lead to cell damage. Second, when the battery voltage drops below the required voltage to actuate the relay coil, the relay would re-engage, turning on other loads in the system, ending in even more severe discharging, and ultimate battery destruction.  Third, if the control circuitry in the BMS failed and cell monitoring was not occurring, the operator would not know because the relay would stay engaged, appearing that everything is working properly. When left unchecked, excessive overcharging could lead to a dangerous battery bank failure.  Normally closed relays are not suitable for BMS use.  The risks by far outweigh the energy conservation.

Bi-stable latching relays use two coils and only require a pulse of current to change state.  With this type of relay, you have the benefit of a normally closed relay in terms of lower power consumption, and the increased safety that in the “OFF” position, the relay doesn’t require power to stay “OFF”.  The downside still exists that if a monitoring/computing system fails, the relay still can give the illusion that everything is working properly (stuck in the “ON” position).  Additionally, compromised wiring to the relay during operation would lead to the system no longer be able to protect itself.  Ask yourself if the savings in energy of using this type of relay is worth the risk; if things fail it could cost your entire battery bank, which is generally a hefty investment. If you did decide to use this type of relay, you absolutely should have a secondary backup method of monitoring and shutting things down if a fault were to arise.

One further consideration is that a relay has a fixed number of life-cycles.  A relay switched under heavy loads will experience wear far quicker than a unloaded relay.  Regardless of relay-style, the massive inrush experienced when re-energizing a system (such as the capacitors in an inverter charging) could easily weld the contacts of the relay together, meaning the relay would be permanently stuck in the closed position, no longer providing system protection.  While a pre-charge resistor could be integrated, very advanced control circuitry is now required to stage pre-charge delays and relay-engagement.

MOSFET-based BMS designs have come a long way, with new models able to handle well over 500 amps of continuous current.  The voltage drop across a MOSFET is quite minimal, and a very small amount of energy is required for MOSFET operation.  The ONLY downside to a MOSFET-BMS is that a high-voltage transient surge can cause premature failure of the BMS.