How much solar to recharge your battery?
It's obvious that you'll need to replace the power you use from your battery. In a lead acid system, this is critical to the life of the battery. In a quality lithium system, this is not critical at all.
If you're using lead acid batteries, you need roughly one watt of solar for every amp hour of battery, just to keep your battery alive. You then need to factor in the amount of power you're using and add this to your solar requirement.
As an example, let's say you have 240Ah of lead acid battery, which means you can use a maximum of 120Ah. If you use 120Ah in a day, you'll have to replace 140Ah. The reason for this is the inefficiency involved in recharging lead acid batteries.
What this means in terms of your daily charging routine is that you need your lead acid battery to be above 14 volts by lunchtime. This is so that the remainder of the day's solar is used to do the absorption stage of battery charging. If you don't reach 14 volts by lunchtime, you're steadily going backwards. In other words, you don't have enough solar and you are damaging your batteries. You also need to hold that 14.4V for at least 4 hours (this is the absorption part of lead-acid charging), if it took more than a few minutes to reach 14.4V. If it only took a few minutes to reach 14.4V, the battery is already fully charged, or it's sulphated (in which case the extended charging time isn't going to help anyway).
There is no need to bring a lithium battery up to any particular charge by the end of the day as they do not lose voltage as their charge drops. You simply need to know your power consumption and you need to be able to more or less replace it each day.
As we've mentioned before (and no doubt will again), lithium batteries do not need to be recharged to 100% each day. So picture that you've brought your lithium battery down to 30% SOC by the end of the day, the weather is likely to not be good tomorrow, so you'll hit the road. You'll only use 10% overnight, so there will still be 20% left by the morning. If you know you will use more than 10% SOC overnight - say 20% - you can either run the car and DC to DC for a while to get back up to 40% before you go to bed, or if you have a Victron battery monitor with the alarm set to 20% SOC, it will need to silenced some time during the night.
Your DC to DC charger will charge as you drive and the solar will still be putting in whatever it can. But it doesn't matter a jot if your lithium battery does not come back up to 100% every day.
It takes some getting used to, and many lead acid die-hards refuse to accept it, but there is far less charging anxiety in a lithium lifestyle!
The lure of air conditioning whilst free camping is often what brings customers to us for a lithium battery install. But you also need to think about how much solar you need to replace the power you use.
We've put together a simple formula based on the advertised capacity of a solar panel, and other factors such as geography and time of year, so that you can work out how many amps your solar should be harvesting to charge your battery.
How to make the most of the solar panels you have, the huge impact that shadows have on your solar harvest, the importance of cleaning your panels and how to make that cleaning last.