Charging Lead Acid with a Buck Converter

I think you might want to do a little more research and build a better understanding of batteries and charging them. any re-chargable battery (such as yours can only be charged so many cycles, you're on your way to frying your battery prematurely, you need more wattage so you can charge a full cycle each time, your set up will charge each day multiple times depending on cloud cover and shade. you shouldn't partial charge and waste your cycles, only charge when you have enough available wattage to fully charge your battery......or just keep buying batteries every 2 months

7AH batteries?

Did you check the solar charge controller for float voltage adjustment pot? Or the equalizing switch?

The new battery is listed at 75AH big change from 7AH.

The convertor is just that, it is not a charger.

The small 7ah batteries may not be a good indicator of how your charge controller will work with the larger leisure batteries.

I see all these trickle chargers on Ebay that just output a float voltage. The don't perform a set charging cycle. My buck converter would be like one of those but more powerful. I am trying to research the batteries and some say that the leisure batteries are just starter batteries and they are crap for my applications and others say that they are just as good as agm batteries. I am really not sure of the best battery to get.

I can recommend from many installations www.solarsunshine.co.uk for solar chargers. You can set the battery type and Ah rating and have a remote control unit.

You need to get some basics on batteries, charge/discharge, sulfodixation, particularly with sealed batteries before you blow any more money, or yourself!!!

Here's a good site to start.

Gassing and venting are a bad sign. You are seriously reducing the life of your batteries. There are plenty of good web sites around that will show you how to charge properly, but to get maximum life, you need to not charge to 100% full.

Lead Acid batteries once over about 70% full, are really very inefficient as well.

Its better to work out what you need in stored power, then buy batteries that are at least 30% greater in capacity and only charge to around 70% (see manufacturers data) as a max.

The plus points are that you will operate in the most efficient part of the batteries capacity, no gassing, no water loss, far less wear and tear.....also you can safely ignore temperature monitoring as long as charge is held to C/10 or less.

Do remember that even then, such batteries need proper ventilation to get rid of any Hydrogen/Oxygen released. No sources of ignition like cigarettes for example....

Why did you buy leisure batteries if they will never ever be discharged.....in such cases, cheaper batteries would have been enough....I also prefer leisure batteries by the way for storage in my caravan....but here they are far more expensive, so you need to look after them and make them last a long, long time.

We have had many blogs on CR4 with regard to charging and maintenance of such batteries.....

These points are very important for any battery charger

1) Peak charging current vs battery temperature graph is very important as there is limit to charge rate that can be dumped in hot condition. If temperature is not monitored then operate at the safest limit.

2) Maximum charging voltage is equally important and this factor is always given by the battery manufacturer and can't be taken as arbitrary number.

3) Buck or boost the voltage (should be a part of the charger input stage) to obtain the right voltage difference between current source input and battery voltage (difference will change as battery gains more charge) else charging will not be proper and this function must be part of the charger itself.

4) Look for these functions in the product features. Some chargers do have fast charging mode and you need to use it only if your charge source lasts for very short time. It is an emergency handling procedure and not meant for regular use. Generally chargers may have multiple charging rates so select that one fits the best for solar systems.

Comment of caution:

Be careful in selecting electronics as many China Electronics in India we find are meant for lasting only limited number of days and they may also set the life of your solar system and battery to number of days. Buy only branded products of high reputation. Avoid picking electronics from streets and don't get tempted just because they are cheap. I am sure they will have no warranty card or not even address of the manufacturer. Not everything from China may be bad so be very selective. Look for product survey report if it is available or dig deep in the catalog if you find one or talk directly to the manufacturer for details.

Don't use a buck converter. Use a battery charger/charge controller of the right spec for your batteries. These will charge differently depending upon the battery's state of charge, with typically three regimes, all of which will avoid gassing, overcharge, undercharge, overcurrent, etc.