I think that I used to know about battery charging but as 40 years go by ...... I need to charge a 75AH battery. In my days it used to be considered that a 10 hour charge/discharge (ie 7.5 amps per hour) rate was most efficient. However I do not believe there is any reason why a battery of this type could not be charged for a longer time at a lower rate (ie for ease 3.75 amps for 20 hours) and still achieve full charge. Is my memory playing tricks on me or did I never learn properly in the first place? Logic behind this question is that the domestic battery chargers appear to have a cut off at 70AH batteries and almost double the price for another 30AH. (I am quoting what the manufacturer states is the recommended highest battery capacity). Would appreciate comments but be kind to an old man!!
What makes you think that this is the place to ask?
See: http://www.keil.com/forum/ - what does it have to do with Keil products?
Are you intending to make amicrontroller-based charger?
Somehow, your memory may be playing tricks on you :)
NiCd and NiMH batteries are normally charged with a constant current, where you may specify that you charge with C/10 for 14 hours to get a full battery. Of course, this has now often been replaced by extreme quick-chargers where you need to constantly measure the temperature and voltage while pushing whatever current you can through them.
But a lead-acid battery is charged by a constant voltage, and the battery will then consume the current it needs in relation to its current charge level and the charge voltage.
So a lead-acid charger normally has a constant-current regulation for very empty batteries, i.e. it varies the voltage to keep a maximum current low enough that the battery doesn't cook. A normal car charger often has a switch where you can change this maximum current - a motorcycle battery requires a way lower max current since it is so small.
As soon as the battery gets charged enough so that the charge current drops below this max current, the charger will switch to constant-voltage charging. This voltage should be lower if the charger is built for constant charging and higher if it is intended for quick-charging of an empty battery. Good chargers can auto-select this, i.e. they start with a high voltage and run for a number of hours or until the current has dropped to a specific level. Then they enters maintainance charging by reducing the constant voltage to a lower level.
What follows from this is that you may not use a high-capacity charger to charge a small lead-acid battery because the max current in the constant-current part of the charge cycle is so high that it will cook the battery.
But you can charge a big battery with a small charger. It will just mean that the charger will spend way more time being current-limited before it switches to the constant-voltage mode, i.e. the charge time will be very slow.
Another thing is that high-end chargers can also pulse-charge the battery. One reason for pulse-charging is that you control the charge current with the charge voltage. So a higher charge voltage gives a higher current. By going for very high voltages in short bursts, you can revive an old and tired battery.
Another reason for pulse charging is that the charge current will slowly reach zero when the battery voltage gets closer to the charge voltage. In short, the energy you get into the battery is much higher for an empty battery than for an almost full. With pulses you can continue to keep up the charge speed all the way until you switch to maintainance mode.
When you get your lead-acid battery, you will often find charge information written on it. If not, then get the datasheet. The charge information will tell what the maximum initial charge current is, i.e. the current limitation to use for an empty battery. The information will also show the charge voltage to use if quick-charging (cyclic use) or if you charge it continuously 24x7. The datasheet will normally improve on that by showing how you can adjust the current and voltages depending on surrounding temperatures. A cold battery can stand way higher charge currents before it cooks.
Look at www.batteryuniversity.com/
This might be getting a bit dated now; eg, there is no mention that NiCd is no longer allowed to be sold in Europe...
I find it a bit cute that the most common li-ion cell used to build battery packs - the 18650 cell - is said to be 18mm in diameter and 650mm long. That is more than two feet long ;)
Yeh sorry to all. My Google search threw up this forum - clearly the wrong place but thanks for the info anyway. Once again my apologies.