A modified sine wave inverter should work.  It will be less efficient, on the order of twice or more the power loss of a DC-DC converter.  It will also be harder on your AC power brick, which is likely to run hotter, whether or not this will cause it to die earlier, who knows.

If you use your laptop a lot (like a navigation computer) it might be worth sourcing a DC-DC converter for it.  We've used boost DC-DC converters for years to power our 20V laptops from 12V batteries.  They seem happier if they are somewhat oversized.  I've used cheap ($5-$10) 150W converter for our 90W laptops.  In an aluminum enclosure with external heatsinks, they have lasted for years, but run warm to the touch.  We have a newer commercial model, but haven't used it enough to know how long it will last.  We also have a very beefy model for running a 3D printer (I have a teen and preteen son, as well as never fully grown up myself :-)  

I've never tried this with a Mac.  Apparently their Mag connectors have an extra wire to see if the power supply is "valid".   With a 24V battery, you would also need a Buck Converter rather than a Boost Converter.  I think a fair number of commercial trucks use 24V, there must be some kind of Mac solution for long-haul truckers.

I did some back-of-the-envelope calcs.  Using an inverter vs a DC-DC converter, you will probably see about 0.4Ah additional power consumption per hour of laptop use from your 24V house bank.  Not enough to worry about if you have a single user and limited laptop use.

Oscar wrote:

Inverter/AC is another question mark... I don't feel like spending money (€200+) on a pure sine wave inverter and I'm unsure how good a modified sine wave inverter would be for the longevity of anything connected to it. Both me and my SO have Macbooks which run off 14-18V so might just build custom power supplies for them. Don't really need AC for anything else. There are these DC supplies for Macbooks but the 1 star reviews there make me feel a bit cautious. Then again I also have a cheap Windows laptop purchased solely for running Delftship which I might want to be able to power... a but a cheap inverter would probably do the trick for this.

I bought this Bestek 200W inverter, £33.10, and it does just fine for charging my Macbook via the Apple mains adapter and iPad and iPhone via the USB sockets.

Yes, Will Prowse suggests putting a thermostatically controlled heater mat under the battery.

Reading through that charging instructions PDF, it would seem that the safest way to charge 2 x 12V Relion batteries in series for longest life would be one stage bulk charging to 28V, putting it at 95% of full charge, and then no absorption. Will Prowse recommends cycling charge and discharge between 10% and 90% of full charge.

Since there's also a high voltage disconnect built into the Haswing motor, it seems to make sense to set 28V as the maximum charge voltage, so that the charger can remain connected while the motor is running without fear of losing power at an inopportune moment.

Oscar wrote:

Horst, I haven't decided on a specific BMS yet but in important thing to look out for (if it applies to your cruising ground) is that it has a low temperature cut-off (usually configured around 5 deg C) as LiFePO4 cells are damaged if charged below freezing. You can test if it works by putting the temperature sensor in ice cold water and measure whether the charging stops.

An alternative to low-temp cutoff on the BMS would be to have it on the charge controller instead, if solar is your only power source. For example Victron sells separate temperature sensors that you can hook up to their products. That said I think it's more sensible to have it on the BMS level so that you're protected whatever charging method you use.

I'm puzzled about how my Relion batteries seem to get around this. The data sheet clearly says that the temperature limits for charging and discharging are -20˚C to 45˚C  and -20˚C to 60˚C respectively. Could there be some self-heating that cuts in at low temperatures? Nevertheless, I should look at the charge controller manual and see if I can set a lower limit.

Nice initiative David!

Horst, I haven't decided on a specific BMS yet but in important thing to look out for (if it applies to your cruising ground) is that it has a low temperature cut-off (usually configured around 5 deg C) as LiFePO4 cells are damaged if charged below freezing. You can test if it works by putting the temperature sensor in ice cold water and measure whether the charging stops.

An alternative to low-temp cutoff on the BMS would be to have it on the charge controller instead, if solar is your only power source. For example Victron sells separate temperature sensors that you can hook up to their products. That said I think it's more sensible to have it on the BMS level so that you're protected whatever charging method you use.

And it's obviously worth being skeptic about super cheap products. There are a lot of good independent reviews of solar equipment (battery cells, BMS's, solar panels etc) on Will Prowse's channel. Electricseas.org as you mention is a good resource as well, many interesting project threads to read through, and the article you linked to seems like a good read, thanks!

I'll try to post here when I figure out what I'm going to buy and why. So far, in addition to the cells, I've also ordered a 24V to 12V converter.