In reply to G0AZS:
You charge lead acid batteries with constant voltage. This is different to NiCd/NiMH cells which use constant current. So to charge a lead acid cell you can use a stablised powersupply. i.e. you could charge up a 12V SLAB from a 13.8V supply you probably already have.
But… 13.8V is not enough to full charge the SLAB., probably you will only get to 75-80% charge with that voltage. The actual charging voltage depends on the use for the cell, a lower voltage is used if you intend to leave it on charge forever and occasionally use the cell. For your use, you can charge the cell with a higher voltage and disconnect when fully charged. In this case 14.3V will do nicely.
The initial current when placing a 7AHr cell on charge will peak high, possibly in excess of 10A or more, but this will rapidly drop. The charge current drops as the cell charges. I use a 2.8AHr cell and after an activation the other day last over 1hr using SSB on HF and a good 30mins of 2m FM nattering, my cell pulled over 5A for 30secs until the current dropped to about 2.1A. It took a couple of hours to slowly drop to about 50mA at which point I removed it from charge.
If your PSU cannot supply the charge current then you will put the cell into slope charge. If your PSU has a current limit, it will lower the voltage to keep the current in range. In this case as the cell charges, the current drops and so the PSU voltage will rise. This is not good for maximum life of the cell. However, if the cell is only in slope charge for a few minutes, it’s not worth worrying about. For a 7AHr cell, you probably want a supply capable of providing at least 3A without limiting.
The other thing to keep an eye on is the temperature when charging. The higher the temperature, the lower the charge voltage. Probably not a problem as long as the temperature is less than 20C. But once it gets above that you need to lower the volatge. The manufacturer’s data shows the derating curve. Most lead acid cells are killed off prematurely by charging at too high a volatge for the temperature. It is what kills off car batteries in hot countries more than anything. Charging at the voltage specified for 0C when the temperature is 30C can reduce the number of charge cycles the battery will sustain by a factor of 10 or more!
Finally, don’t leave the cell discharged. Always charge it up fully as soon as you can. Otherwise the plates will sulphate and you will find you cannot charge the cell or the capacity drops to a fraction of its original capacity. You should charge up the cell every 6 months if you are not using it as it will slowly discharge and start to sulphate.