Building a Battery Charger

[fatalelieberi]

Hi experts in EEVBlog,

As a newbie in electronics, I am currently interested in building a battery charger, specifically for a car battery (lead acid). From what I learn about battery charger, it maybe similar to an adjustable DC power supply, but with a preset of voltage (maybe about 13.8V-14.4V) and a constant current. I have studied the behavior of charging a battery. Basically, it will have 3 phases: at phase 1 the battery will pull all available current until it reaches the topping voltage (it will be limited with a constant current circuit), at phase 2 the battery will draw less current and it bottoms out, and at phase 3, it will draw very low current to counter the self discharge current.

I've experimented to find if this is true by charging a small car battery (7Ah rated) with my DC power supply. I set the power to 2A and 13.8V. So far, it behaves similarly like the theory suggests. However, when I searched through the internet, I found that there are many car battery chargers out in the market with a very high current rating, such as 12A, 15A, 20A, so on. What bugs me is that how does this current rating affect the charging of a battery with a far lesser capacity than the battery charger? Will the battery pull all available current that the charger can supply and potentially damage the battery because of over charge? I need to know this behavior to design my own battery charger, and how to sense the state of charge of the battery. Any explanation is very much appreciated.

[Paul Price]

You should read more about float charging a lead-acid battery.
You should not allow your charger to apply more charging current than the battery you are charging can handle.

You can charge a larger capacity at any current level less than it's max rating, charging al low current will just take longer, but has the advantage of being cheaper to build and less likely to damage the battery being charged.

It is you goal to charge the battery as quick as possible then you design the charger to deliver the max charging current the charged battery can handle and use current limiting to prevent overloading the charging power supply or your battery.

[fatalelieberi]

Then, that means, the battery chargers in the market right now are able to sense the battery capacity so they can deliver current under the battery max current rating. Is that right?
 

Now, I'm really curious how they do it, measuring the battery capacity it's charging. Because, you see, the battery charger can be applied to any battery sizes, there's no guarantee that the users will use the proper battery.

Can you give me some revelations?

[Ian.M]

A smart charger can control the maximum current to limit the rate of voltage rise. and roughly estimate the capacity from the rate of rise, and estimated initial state of discharge, approximated from the initial terminal voltage.   However there is usually some user configuration involved to select the correct lead acid technology - traditional flooded with top up caps, flooded maintenance free with lead calcium alloy plates, or gel-cells (AGM) as they have different maximum charge rates, end of charge voltages and float voltages.   A not so smart charger may simply be sold as only suitable for a range of capacities e.g. 10Ah to 1000Ah and have a warning in its manual that its unsuitable for charging gell-cells or smaller batteries.

One thing to be very careful of - Some makes of bench PSU can be damaged if they are switched off or loose mains supply with a battery still connected.  Its generally advisable to use a series diode when charging a battery from a bench PSU.  Unfortunately that makes it difficult to set the correct voltage for the end of charge float voltage because the diode Vf drop varies with the current, and with the battery partially discharged you've got no way of knowing the final current to maintain the float voltage.

For Lead Acid batteries, if you set up the PSU output voltage, with a load resistor on the diode in place of the battery to give a current of C/1000 (that is battery capacity in Ah divided by 1000h to give a current), e.g for a 100Ah battery and a 13.8V float voltage, you'd want 1380 ohms - 1.5K would be near enough, then adjust the PSU so you get the desired float voltage across the resistor. it will be safe to leave it to charge to completion and even float for a few days.  However, for longer term float charging you should re-adjust the PSU output voltage to get the correct float voltage for your battery at the average ambient temperature, as C/1000 is just a guess at the maintenance current.   

You can charge several Lead Acid batteries off the same PSU with a separate isolation diode for each.  The lowest will get the charge until it catches up with the next lowest and so on till they all reach the float voltage together.  The max current must be set to be safe for the smallest battery (usually no more than C/5).   If you keep batteries on float like that long term, check the current regularly and that the float voltage is being maintained as if one battery develops a shorted cell, it will hog all the current and drag the float voltage down so although the diodes will protect the other batteries from immediate damage, they aren't getting the maintenance trickle charge they need  for long-term storage.

Charging NiCd and NiMH batteries off a bench PSU without damaging them is trickier and unless you know *EXACTLY* what you are doing and have a circuit to isolate the battery on excessive temperature rise or at the correct final voltage, charging any sort of Lithium cell can be dangerous and charging multi-cell battery packs without a cell balancing circuit is high risk.