Hi Friends! I read many comments on battery charging and I thought I might start a new thread combining the various contributions(those I can recall). I will try to discuss fundamentals.
There is the famous question: Which is better, a constant voltage or a constant current charger?
Nowadays, it is easy to make either, in fact it is using the same IC voltage regulator set as a constant voltage or current source. Lets see.
The voltage will have a maximum the current will taper down.
The current will be the same throughout the charging period, even if the voltage rises.
Let's start with a Lead Acid Battery. That is the battery in the car, in the bike, and and in the emergency light. Constant voltage(shall I say peak voltage really) is normally used to charge these batteries. That is perhaps why the device used with the alternator is called a voltage regulator. It is critical that the V does not exceed 13.8V. If you have noticed the amp meter in a jeepney, it is zero if the jeepney has been running a while. The battery is fully charged. We can use a volt meter such as those in sport cars. And I recommend we should. The AMP meter does not tell us if the voltage is max at 13.8V.If the voltage regulator is faulty and there is over voltage, accessories can be burned out and the battery destroyed. They say that why Pinoys still use an amp meter is that it is because he started with it in the U.S. army jeep. Obviously, the advantage of a max voltage charger(I am now using the term) is that you can leave the battery hooked up and forget it. The plates won't boil. The VR watches the V and as it reaches the max, it shuts down the current and stops charging.
In fact in Lithium based batteries, it is a complete NO NO to exceed voltage (4.2V per cell for Ion and Poly).
Same with nickel based batteries. At the moment, I cannot think of a battery type that is not voltage sensitive.(They are all current sensitive too!)
Why then all these things about constant current charger and some mags will say that is the way to charge nicads. Well here are some rules of thumb.
1. A charger at .1C ( 10% of the capacity, minus the H), is harmless for most batteries,(lithiums do not like trickle charging).(Many commercial car battery charger is rated at 4 amps since the smallest car batt is the 1SNF which is a 40AH batt.Ten percent is 4 amps!).
2. The formula for charging period(or how much Mah to put back is capacity(fully discharged) is plus 50%. (you an use a higher C rate or a longer time to charge).
Let's now talk of the nickel batteries(nicads, and metal hydrides). Let us say you have a 1000 mah cell and it is 1.1V or less. The safe charger is a 100 ma charger and 10 hours would have injected 1000 mah. Leave it there for another 5 hours, so 15 hours. There is no need to put a timer cuz it's only a .1C charger. However, 15 hours is too long for some users l like us modellers. So, we raise the C, and generally 1C is max as a general rule. so, charge it at 1000ma or 1A. Leave it there for 1.5 hours. Now, this is where the problem begins. You should not exceed the 1.5 hours. So put a timer. In RC cars, this formula was disregarded and nicads were abused. The modeller would discharge the cells of 6 or 7 to 1.1V per cell (or sometimes zero V), then charge at a rate so that the V goes up to 1.35V per cell in 15 minutes! Waaaa....the battery heats up and a fan is used sometimes. It is charged at more than 4 amps! and ah, this is where constant current comes in. If you had used a tapering current charger, you will not put back 150% of the spent capacity using the formula above. The average then assuming the current drops to zero at the end of the charge is just 50%. So, bottomline, make a constant current charger in this case but watch it! Use a timer or kaput, you will have a fried battery pack.
So, it deciding which mode to use in your lab, take the above precaution. And for an average fellow user, he may not want to bother with spent capacity(your batt could be partially discharged only). So give him a .1C constant V charger. Most wall chargers that comes with our TX in RC has a simple transformer cum diode charge which current tapers down as time goes by. In America, authors might call it the charger for Dummy's or even an Idiot's charger. Now, for the sophis....ah, he has the gizmos to check V, I, capacity etc...and he is in a hurry, so....constant I charger is his choice. But beleive, he will give someday and go for the simple. Today, the sophisticated charger will cycle your battery from once to 3 times. I have one, the TLP 4000 (and it used to cost P4000 too). It will dischrge the nicads or hydrides, fast charge it, slow charge it, dischargers it again etc...Most use the delta peak detection system to detect when the cells have reached full charge. Finally, it switches to trickle charge .1C or less. A pulsing current works better than a steady DC to charge the nicads. They are called pulsed chargers. Some circuits even introduces some AC into the charger. Some would even charge 2 units, discharge 1 unit ala cha cha.
Use the contant V charger. It is friendlier to the battery. If you use more than .1
C, put a timer. And since the current will taper, start at a higher level so the average is what you want.
How to make a constant V or constant I ckt., google the datasheet of the LM317 and the 78 series(78XX, XX is the voltage). For higher current, use a bypass transistor, a 3055 or a 2955).