Imhotep's Lab Interactive FAQ

I currently charging originally bad 12V 7.2 AH battery to what I consider as rapid charging. The voltage increase very fast.

I put a fan to the battery and leave it running yesterday. The voltage drop to 4.4 Volt in the morning.

Then I charge the battery using joule thief version of Imhotep Radiant Oscillator. I use maximum BEMF that my circuit allow by adjusting the frequency. The input is 12V 300mA (measured) from computer power supply.

After two and a half hour, I put the fan in the battery again and then measure the voltage. It become 7.4 Volt.
3 Volt increase in two and a half hour = 1.2 Volts per hour.

I see that most of you charge the battery in two days. While mine seems to be charging the same rate as cellphone battery charger.

Is it wise to charge the battery in this rate?

I am affraid to damage the battery or something. The charging is definitely not surface charging since I can use it to power computer fan long time without too much voltage drop.

I think you could be making some mistakes.

When you discharge a lead acid battery more than 10% of its fully charged voltage you are damaging the plates. and will shorten the battery life.

Always measure the voltage when the battery has rested for an hour or two

eg.
Charged voltage = 12.5v
10% of charge - 1.25v
when discharging do not let the voltage drop below 11.25v

I personally only discharge 5% on starter batteries.



Charge and discharge rates should ideally be in the C20 range, that means take 20 hours to charge or discharge or longer, faster than this will start to deteriorate the plates.

Starter batteries will stand higher discharge and charge rates but even so the life will be shorter. Starter batteries are the type used on cars and motorcycles. Deep cycle batteries are damage easily on fast discharge rates.

There are three common, lead acid, types of battery.

1, ordinary type that has the plugs where you top them up with distilled water.
2, Lead calcium batteries, Usually sealed but it is often possible to find a way into them.
3, Gel type, these are sealed and use a type of gel instead of normal acid.

All three can be found in starter or deep cycle applications

Ordinary batteries are very easy to monitor because you can get access to the plates and you can check the specific gravity of the acid. Lead calcium batteries work at a slightly higher voltage than ordinary batteries (0.4 to o.6v higher) but are otherwise more or less the same. Gel type batteries, I have found, can be damaged with high charge rates with a radiant charger. I don't really know the reason, just that I know that all 5 of my gel type batteries have now failed.

Check out my posts in this section.

I don't know the type of battery you are using but rapid charging and reducing capacity is a sign of a failing gel type battery.

Having said all that if you have discharged your battery down to 4 volts the initial rise in voltage will be rapid but slow as it gets to around 10 or 11 volts

i would expect a 7.2ah battery to charge in around 24 or 25 hours.

This is just a guide because charge rates often increase with frequent charging using radiant energy, suphated batteries take much longer. 7.2 / 0.3 = 24 hours

Thank you . The information really help.

Since it appear that the battery have the behaviour like you mention, I currently chaging it with even faster rate and see what happen. It seems to increase the surface charging part. Currently measured as 12.8 Volts.

Hey Mick, as you may remember, I have been buying Deep Cycle Marine (Interstate)
batteries (Group 24 automotive size), I was wondering about some specifics for a
battery of that type. They told me it was 65 AH, but I have been unable to find that
info on my own from Interstate's literature. I'm sure it is pretty close, though.
You seem to be the #1 battery guru on here my friend! Please help! Would you
discharge a deep cycle more than a starter battery? What specs would you use?

I am not familiar with your battery.

True deep cycle batteries are very big compared to a starter battery of the same amp hour rating. The plates are thicker and more robust and there is also a larger reservoir of acid. Because of this they are more expensive too.

Marine batteries, i believe, are heavy duty starter batteries ie closer to a starter battery than a deep cycle battery. I am sure there are some differences to make them more suitable to a wet environment.

What is important for a starter battery is cranking amps. ie how many amps it will give out in a near short. Deep cycle batteries perform poorly at this.

The capacity of the battery is limited by the amount of chemicals that are available to react.

If we take a starter and a deep cycle battery of the same capacity what is the difference?

Starter batteries have a large surface area to volume ratio but this makes the plates mechanically week when discharged but good at dissipating heat. Plate failure is a real possibility when you draw off full capacity from a starter battery. However you can draw a high amp drain without damage.

Deep cycle batteries have much thicker plates so there is more material to maintain structure of the plates. because the surface area is smaller heat will build up when they are heavily charged or discharged which can cause plate failure (melting). You can draw the full capacity of the battery without risk of damage to the plates.

If we are using batteries in a cycling operation and drawing close to the batteries full capacity in each cycle, a starter battery will fail in a year or two. so if we only draw half its capacity we will be leaving more of the plates for structure and it will last longer.

A deep cycle battery will last much longer and so work out cheaper to use in cycling operations.

The usable capacity of battery is found by measuring the total amount of power that can be drawn when we discharge it. we do not draw the battery voltage too low because this causes damage. In the old days it was considered that b battery was fully discharged of usable power when the battery had lost 10% of its voltage. There is a good margin of safety in this.

To make starter batteries lighter, smaller and cheaper this margin of safety has been compromised. so what voltage do we drain our batteries down to?

If we are using true deep cycle batteries the 10% rule is fine

If we are using starter batteries and want to have a reasonable life i have found we need to draw less than 60% of its capacity or 6% of its voltage, I now only draw 5% to give a margin of safety.

sorry this is a long post but i wanted to give some background on why batteries fail