Lithium 12V battery test
Prices are falling for 60-120Ah lithium batteries with integral charge monitoring. Emrhys Barrell puts 12 units to the test
PBO exclusive: a dozen 60-120Ah lithium batteries with integral charge monitoring compared
Boat owners have long relied upon the lead-acid battery to start their engines, run electric lights and, these days, to power inverters to run household items such as kettles, microwaves and more. Yet lithium batteries offer much better performance and lifespan, but at a much higher up-front price. So is it time for you to switch from the 19th century technology to the 21st?
The generally accepted rule of thumb with lead-acid is that your battery pack capacity should be double what you are likely to use in one day, or between recharges. This is because most lead-acid batteries should not be regularly discharged to below 50% of their capacity, or their cycle life will rapidly diminish. Some lead-acid technologies will improve on this, but at greater cost.
At the same time the faster you take power out of a lead-acid battery, the less overall capacity you will get. Typically a battery that will give you 100Ah if it is discharged at 5A, ie over a 20-hour period, will only give you 80Ah if you discharge at 20A, ie over 5 hours, and 50Ah if you discharge at 100A, ie over a nominal 1 hour. This becomes especially significant if you are powering an inverter, or driving an electric boat.
Thus your nominal battery capacity will have to be even greater if you are regularly taking out high currents.
In addition, the number of useful cycles any battery will give you over its lifespan is governed by how low you take its charge each time, known as Depth of Discharge (DoD), and how fast you take the power out. The cheapest lead-acid leisure battery on sale in your chandlery will probably give you at best 100-125 cycles at 50% DoD. Regularly take this down to 80% DoD and the figure will drop to 25-30 cycles, at which point you will probably only have 50-60% of the original maximum capacity remaining. If you take the capacity down to 90% DoD – when the lights start to dim – you will probably get 10-20 cycles at most.
Better leisure batteries on sale will improve on these figures, but usually not by much, and you’ll generally have to look to traction batteries, designed to power electric vehicles, to get over 500 cycles at 80% DoD. Deep Cycle batteries from Vetus, and Lead Carbon batteries from DBS Leoch both claim even greater cycle lives.
And the final bugbear with a lead-acid battery on your boat is sulphation. Leave it over winter at 80% charge, and that will become its maximum capacity, no matter how much you charge it up afterwards. Do that two years running and you are down to 60%.
Lithium batteries claim to turn all these performance expectations on their head. They claim discharge capacities of nearly 100% even at 100A discharge, and cycle lives of 2000+ at similar discharge currents and 80% DoD.
The aim of our exercise was to put these claims to the test.
To ensure fairness across the range we charged each battery on test with an approved lithium charger – either the one supplied with it, or with a Victron Smart Charger, or Mastervolt ChargeMaster Plus. We then carried out three measured discharges (with recharging in between each) and measured the Ah they delivered: 1. At 25A – using an MK constant current discharge tester 2. At 100A – by connecting each to an inverter, with a 1kW electric heater plugged in 3. At 150A – as above but with a 1.5kW heater 4. Finally we measured the maximum current they would deliver for 5 minutes, on the grounds that this is what you would use to boil a kettle or power a microwave.
Some batteries being tested only had a 90Ah or 60Ah rated capacity, so we only took 90A and 60A from these to test actual performance compared to claimed performance.
Some batteries could deliver the 150A, some could not. In all cases it was the internal battery management system (BMS) that determined when they shut down, with different manufacturers choosing different cut-off points that they thought would enable their unit to deliver the best long-term results and cycle life.
The temperature of our test facility varied between 10-15°C. Most makers’ stated capacities were achieved at 25°C, so our results were bound to be slightly lower than the claimed figures, by approximately 2-4Ah.