Big Bertha gets a Lithium Battery

Big Bertha gets a Lithium Battery

Big Bertha came with Absorbent Glass Mat (AGM) lead acid batteries when we bought her in 2018 and we replaced these with another set of AGMs in Namibia in 2022. They have served us well. Lead acid batteries are very robust but have two inherent disadvantages in our application:

• You can discharge them to no more than 50% without permanently reducing their capacity and/or shortening their life. This means our 200 amp-hour (ah) battery bank only has a useable capacity of 100ah.
• Their internal resistance increases with charging. This means that charging them from say 50% State of Charge (SoC) to 80% SoC is relatively easy, but from 80% to 100% SoC is painfully slow. This is particularly problematic when charging from our genset.

Lithium Iron Phosphate (LFP) batteries do not have these disadvantages; they can be discharged to 0% (realistically 10% to avoid power outages) without impact on their capacity/lifetime and have a very low internal resistance, meaning they accept a high charge current right up to 100% SoC.

But they have their own disadvantages:

• You must not charge or discharge them outside specified temperature ranges.
• You must not exceed a specified discharge current.
• You must not over charge them.

An infringement of any of these limitations will likely damage the battery. This requires management by a Battery Management System (BMS), often internal to the battery.

Dave decided to upgrade to a 24V 314ah LFP battery instead of continuing with AGM batteries for several reasons:

• The cost of LFP batteries has dropped significantly; the price of replacement high end AGM batteries exceeded the price of our new mid level LFP battery (WattCycle 24V 314ah).
• The combined weight of our two 200ah AGM batteries is 120kg (yielding 1.7 ah/kg); the LFP 314ah battery weighs 70kg (yielding 4.5ah/kg).
• The LFP will provide a usable capacity of 300ah compared to a usable capacity of 100ah for the AGMs.
• The LFP will charge far more quickly.

Supporting modifications:

• BB is currently set up to charge the house AGM batteries directly from the alternator via a Voltage Sensitive Relay (VSR). Because LFP batteries have very low internal resistance, this would stress the alternator by drawing a high current. Instead Dave has installed a DC/DC charger that regulates this current and has a LFP battery charge profile.
• BB’s other chargers (solar, inverter and shore power) also required re-programming to LFP charge profiles.
• Finally, BB’s battery bay needed adaptation to house the new battery size.

Initial Impressions

The LFP charges fast all the way to 100% SoC – there is no slowing down over the last 20%. The DC/DC charger can provide 50A and if we have discharged to nearly empty, charging will take 300ah/50A = 6 hours to charge. A day’s driving. We normally only discharge to 85% over night, so that will be 45ah/50A = 1 hour’s drive!

The LFP will come into its own on a dark dingy day when the solar charging is marginal. We should be able to survive two days without charging now. And if stationary, the petrol genset and inverter/charger will recharge the LFP in no time. We will report back later…

Thank You

We have to thank Fabi and Chris for being able to stay with them in Las Vegas. We we were able to put our food in their fridge, stay in their house, use Chris’s workshop and Chris helped manhandle the heavy batteries. It would have been a difficult task without their help.