Hi, I posted the following thread: http://forum.allaboutcircuits.com/showthread.php?t=98886 and received a lot of informative comments but realised that I was asking too many questions at once, so I decided to separate it a bit.

When charging deep-cycle lead acid batteries, there are usually three charging phases (bulk, topping and float). During topping and float, the charging is normally done at a restricted power level in order to maximise the amount of charge put into the battery, since the temperature/resistance can go up if it is exposed to too much power. However, during the bulk-charging phase (e.g., 0-70% charge level), the idea is to get as much power into the battery as it can take according to its limitations.

1. Since the battery is like a resistor with a predictable value depending on the charge state, therefore the battery determines the voltage at which it will pass a particular current through the circuit, or the current at which it will maintain a certain voltage difference between charging voltage and terminal voltage (i.e., this voltage difference being what is multiplied by the current to calculate power going into the battery). Correct me if I am wrong here.

2. Is it true that for the bulk charging phase, the current OR voltage must be constant, such that: if it is constant current, the charge level is read by voltage feedback, and if it is constant voltage, the charge level is read by current feedback?
Or is it true that current and voltage can both change during this phase and still maintain a coherent relationship that can be used (perhaps together with temperature as well) to estimate charge level?

Let me know if I am not clear, and thanks for the help.

Billy

 

1. Right

2. Neither MUST be constant. But the typical arrangement is a charger that could, if uncontrolled, supply so much current that it would blow the battery or itself. To prevent that from happening, the controller holds the current down to the specified level. So there's your constant current. This phase ends as soon as the voltage rises to the specified limit.

I believe a smart controller could continuously vary the current during bulk charging and still get the job done. I imagine the voltage set point for stopping bulk charging might change for different current levels.

 

Usually both are controlled. During initial charge the current is limited to the battery capacity divided by 10 (C/10) and the voltage set to about 2.36 volts per cell. If the battery wants more current than C/10 the voltage will drop because it is being current limited. Once there is enough charge that the battery draws less than C/10 the voltage control takes over and a constant voltage is across the battery (say 14.2 volts). As the battery charges the current will go down as the battery voltage approaches the charger voltage. When it goes down to about 3% of c/10 the voltage can be lowered to around 13.6 for a float charge.
This is actually easier than it sounds for small batteries.

 

Thanks @ronv and @wayneh. So from moment to moment there is a steady relationship between current and voltage that can be used to determine the charge state? I.e. there is no significant compensation that has to be made to the charge state calculation depending on the amount of power going into the battery?

 

You may find this informative:

http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

 

If the battery internal temperature isn't really close to 25°C/77°F, you should compensate for the difference. the temperature coefficient is roughly -3mV per cell per degree C away from 25°C, so for a 12v cell that has 6 cells, that's -18mV per °C away from 25°C. Failure to compensate will result in over- or under-charging, and will shorten the life of the battery.

Old-technology 'standard' lead/acid batteries should be charged relatively slowly; C/10 or less to avoid excessive heating. AGM batteries can be charged as fast as C/3, sometimes faster; see the manufacturer's datasheet for specific recommendations.

The warmer a battery is, the more chemically active it is, the greater the potential energy storage/release, but the shorter the battery life. A cold battery will last considerably longer, but won't have the energy reserves that a warm battery does.

Automotive batteries are designed to output hundreds of Amperes for very short periods of time (a couple of minutes tops) and then be immediately recharged. Discharging this type of battery deeply will cause it to fall apart inside very quickly. The plates are very thin, and while this gives them lots of surface area to quickly release/accept energy, it makes them mechanically less robust, and more susceptible to corrosion.

'Marine' deep-cycle batteries aren't really deep-cycle batteries, they are sort of a hybrid between automotive batteries and true deep-cycle batteries.

Basically, any lead-acid battery discharged past around 30% repeatedly will have a much shorter life than one that is always kept charged.

A battery that is deeply discharged should be charged relatively slowly until it's above 11.5v; a battery lower than that might have a shorted cell, and dumping lots of current through a faulty battery may cause cells to rupture/burst, and acid spills.

 

Thanks SgtWookie, I knew there was something going on as I keep seeing it written that you should leave a battery for a while before testing voltage. Must be the temperature then. Might add feedback from a thermistor on the terminal lug.