Q Can I fit new lithium iron batteries easily?

Are lithium batteries really as ‘drop in’ and ‘plug and play’ as claimed? I would like to replace my two 100Ah Numax batteries with a single 200Ah underseat battery when they come to the end of their useful life.

This would give me extra payload, extra storage, more usable power and a more stable supply to my 1,800W inverter and even save me regularly hooking up at home in the winter.

But the alternator, mains PSU and solar panel bulk charge at 14.4V and 12V lithium really needs 14.6V. If 14.4V means lithium only charges to, say, 90% then that would be OK and might even extend its life.

Each of these power sources also has a float charge voltage of 13.8V. And, as I understand it, lithium batteries neither require nor like a float charge. So when I am hooked up, should I be turning the on-board charger off until the lithium batteries are depleted (and they all seem to have a coulomb counter inbuilt for the purpose), and should I normally be leaving the solar panel switched off except when there is a load (like the compressor fridge)?

Finally, what about the alternator? My ’van is based on a 2013 Ducato 3L Maxi. I don’t know the output of the alternator. The BMS of the battery should limit the charge current to 100 or 120A depending on battery choice, but is this likely to lead to premature alternator failure?

Mike Clark

AMost purveyors of these new batteries claim they are easy to drop in. They are, without doubt, very beneficial to our leisure activities by reducing weight, increasing available electrical storage capacity and having a much longer service life compared to lead acid-based alternatives. But at a price. Having said that, a 100Ah LiFePO4 battery 18 months ago was £1,000; the same battery now is closer to £650.

Yes, the ideal maximum charge voltage is 14.6V, but most give a tolerance of +/-0.1 or +/-0.2 volts. And, yes, some claim that erring on the lower side of charge voltage will only marginally reduce the maximum energy charged, but will increase service life.

The problem is that this technology is relatively new in our industry and no one has had it long enough to test the claims for longevity. These lithium devices do have electronics inside them and this governs the maximum and minimum operating voltages and also the maximum charge and discharge currents.

We seem to get away with a float charge voltage of 13.8V. If you have a high-power inverter or a fast battery-to-battery charging unit then you must ensure that the current rating of the BMS (battery management system) is in excess of these. Just because the BMS may be rated at 150A does not mean that the battery will draw 150A from your alternator. The alternator will always look after itself as it self limits.

We’ve had our own lithium unit for four years without any hitches. We have a 45A B2B and 1,800W inverter. The 150AH battery has a 150A BMS, with the charger setting as AGM so 14.7 volts max then falling back.

Clive Mott