Storing Outside and Solar charging

[IanRutherford]

Hi Forum,

Aplogies if this has been asked a million times before...

I need to store my 2021 Knuas Boxstar for 6 months over winter - in a secure compound - not at home.  We have sold our house and will be abroad so regetfully I will not be able to attend to it.

I have a 100 Watt Solar panel and battery Master fitted.  My question is, if stored outside will the solar panel be enough to keep batteries topped up?

If not, I either need to find a facility that will allow permamnent plug in, which I imagine would be expensive or remove the batteries and keep on charge elsewhere. 

Any comments or recommendations gratefully received.

[Labby]

I have the same size solar panel and a battery master fitted to my van and the batteries stay charged over the winter but it may depend on the part of the country you live and how much daylight you get on the panel. (I live on the south coast  and there is nothing in the way to cause shadow across the panel)

[Steve928]

Also consider that in 6 months unattended the panel could become dirty enough to seriously reduce its output. Personally I would just disconnect the batteries and leave them in place. It's what I've always done with boat batteries over winter storage and (as long as they are in good condition) they lose surprisingly little charge.

[weldted]

Batteries dot like the old if its the engine battery in its metal box I would remove it and take it home if storing on concrete put some wood under it. Make sure it is fully charged before you leave it.

[plwsm2000]

If you have an alarm / tracker, removing the batteries may not be an option.

If you are technically minded, it is possible to get an approximation of how much solar energy you can expect over the winter period using this online calculator. https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#api_5.2

You can enter the exact location (anywhere in Europe) where your motorhome will be stored.

The calculator is intended for larger solar systems but the results can be scaled down quite easily. In your case, set the kWp to 1 (=1000W panel) and divide the results by 10 to simulate a 100W panel. 

Assuming your panel is on a flat roof, set the "Slope" and "Azimuth" to 0.

After pressing "Visualize results", you can hover the mouse over each month to give the average kWh for that month. Although you can work out the total power over say a 6 month period, you just need to look at the lowest output months (Dec/Jan in the UK) to check you have enough solar over this period to prevent the battery from discharging too much. If you get enough solar energy in Dec/Jan, you don't need to worry about the other months.

You need to check that your battery will last a reasonable time with no solar. This is based on the battery capacity and the normal current drain. You don't want to over discharge the battery, so use 50% of the battery capacity (in Ah). 

As a very rough approximation, the drain might be around 50mA (0.05A) for the engine battery. You should measure the drain to get an accurate value. Assuming you have a 100Ah battery, this might last about 6 weeks before the battery gets to 50% (100Ah * 50% / 0.05A = 1000 hours). 50Ah over six weeks is equivalent to 33Ah over 1 month. Assuming you need about 14V on the battery, this gives 462Wh (33Ah * 14V) per month drain.

Now that you have calculated how much energy you need to keep the battery topped up (in Wh), just look up the expected solar energy in Dec and check it is higher that this figure. (Obviously you need to combine the total drain on both batteries if fitted)

The attached output shows that you could expect 1481Wh in December if you were in central London. (14.81kWh = 14810Wh for a 1kW panel so 1481Wh for a 100W panel). This has a decent safety margin from the required 462Wh.

Be aware that the calculations assume no shading on the solar panel. This can make a big difference in the results. 

The above method is only useful over longer periods and where the battery current drain is roughly constant (i.e when in storage).  If your current drain is high and your battery will only last 1 or 2 weeks, you will need a much greater safety margin. The "month average" figure should not be used to calculate a daily average (you might get a couple of weeks in the month with almost no solar) 

 

[IanRutherford]

Ma y thanks people for your replies on this matter - most helpful