battery charger mode explanation

[motion30]

I have a magnum charger inverter and a small master volt 40 amp charger both charging a house Bank consisting of 5. 8 D batteries. nothing is wrong I'm just trying to understand how the charger works. when I start my gen set in the morning the Magnum panel reads balk. charging, hundred and five amps or so , then after awhile it goes in to absorption charging mode still over 100 amps so if the amp input into the batteries is the same what is the real difference between balk and absorption mode ?

[noelex 77]

When switched on the the battery charger will put in the maximium amps untill the bulk voltage is reached. This is caused the "bulk phase". It is characterised with a constant current and rising voltage.
Most battery chargers then switch over to the absorption phase. Here the battery charger will attempt to keep the voltage constant (this is now called the absorption voltage, but this is usually exactly the same as the bulk voltage). As the battery becomes more charged the charger will reduce the current, but keep the voltage constant.
This absorption phase will continue, usually for an hour or more, than the charger will switch to lower float voltage.

As example if the bulk and absorption voltage is 14.6v and the float voltage is 13.7v
When the charger is first turned on the voltage will gradually rise from the resting battery voltage of say 12.3v up to 14.6v. This is the bulk phase.
The charger will then maintain the 14.6v (typically for one to two hours) this is the absorption phase
The battery charger will then drop the voltage to 13.7 (the float phase) and maintain this voltage untill a new cycle is started (usually the next day)

[rebel heart]

Just to add to that, one of the more annoying aspects of battery charging is that it takes a lot longer to get that last 10% into the batteries. If you have solar (or wind) realize that towards the end of your day your charge controllers will be rejecting a lot of inbound (free) electricity.

As a general rule of thumb if it's sunny outside and you see > 13.8 volts, it's a good time to plug other things in that are power hogs (charge the laptops, etc).

[motion30]

Thanks.guys. I agree both bulk and absorption modes act similar and I am planning on a good size solar a rray but I have to wait to replenish my cruising kitty

[sailinglegend]

Originally Posted by noelex 77

....As the battery becomes more charged the charger will reduce the current, but keep the voltage constant......

The charger doesn't reduce the current - the batteries limit how much current they will accept at the "Absorption" voltage. Raise the Absorption voltage by 0.2v and the charge current will rise. Lower the Absorption voltage to a much lower "Float" voltage of say 13.2v and the current will reduce dramatically. The problem is most chargers/controllers drop down to Float much too early because they don't know the size of your battery bank, or how many of the amps they are producing are going into charging your batteries or powering your fridge.

At the start of the Absorption phase the batteries may be 80% charged, at the start of the Float stage the batteries may be 90% charged.

The Absorption phase may be 5 x the Bulk stage, so getting the battery back to 100% when at a low "Float" voltage can take many, many more hours.

See this diagram of a multi-stage charger/regulator:

[noelex 77]

Any charging algorithm is a delicate compromise.

The settings need to be different or different batteries and often the ideal compromise is different when charging via a generator (where the batteries need to charged as high as possible in a short time) verses shore power (where the batteries are on charge for a much longer period).

Unfortunately not many battery chargers are adjustable enough to adapt ideally to these different applications.

The method of terminating the absorption stage is also important. There are many different ways of doing this. The more sophisticated and adjustable systems (when correctly adjusted) do a much better job and have an impact in extending battery life.

[noelex 77]

Originally Posted by sailinglegend

The problem is most chargers/controllers drop down to Float much too early because they don't know the size of your battery bank, or how many of the amps they are producing are going into charging your batteries or powering your fridge.

Chargers using battery return amps alone to terminate the absorption stage will terminate incorrectly with any load on the system. (Unless they are fitted with a shunt close to battery) This is common problem using car battery chargers for a marine system. Car battery chargers assume the battery is being charged with no load. These chargers are also calibrated for a small sized battery as commonly fitted to cars.

However both of these faults will overcharge, not undercharge a larger marine battery with some load.
The charger will drop down to float too late not too early.

Originally Posted by sailinglegend

so getting the battery back to 100% when at a low "Float" voltage can take many, many more hours.

Getting a lead acid battery to 100% SOC does take a long time even with a very aggressive charging algorithm.

[goboatingnow]

The problem is most chargers/controllers drop down to Float much too early because they don't know the size of your battery bank, or how many of the amps they are producing are going into charging your batteries or powering your fridge.

Correct and its a major problem, resulting in undercharged batteries and misleading 'battery charged' lights on chargers.

The charger has only two measurement metrics of concern. Charger current and terminal voltage. Most chargers simply exit absorption mode on a current set point, ( with a overriding safety timer) hence they get confused if supplying a load as well, as usually the timer trips the charger into float mode and the in effect the battery now just trickles charges to completion.

Some chargers also use di/dt as a termination method, but in reality it doesnt do a better job.

IN reality chargers in a load sharing environment, should really have a loads current sensor so that the charger can distinguish between battery load and system load currents.

IN reality battery charging should be done with no load on the battery, in practice not so easily achieved .

Dave

[noelex 77]

Originally Posted by goboatingnow

Correct and its a major problem, resulting in undercharged batteries and misleading 'battery charged' lights on chargers.

The charger has only two measurement metrics of concern. Charger current and terminal voltage. Most chargers simply exit absorption mode on a current set point, ( with a overriding safety timer) hence they get confused if supplying a load as well, as usually the timer trips the charger into float mode and the in effect the battery now just trickles charges to completion.
Dave

I am sorry, but this is the wrong way around.
If the charger uses current set point to terminate the absorption charge it will terminate later, not earlier with a load.

Suppose the current set point to terminate absorption is say:
Less than 3amps at the absorption voltage.

A three amp load will cause this set point to never be reached. (If its measured at the battery charger not an external shunt). A current based termination of the absorption phase will overcharge not undercharge the battery (unless it is measured at the battery itself, when it will terminate correctly. Unfortunatly this is rarely done)

As you say to help overcome this some battery chargers also (or in some cases only) use a timed absorption phase that will kick in before too much overcharging occurs.

[goboatingnow]

Originally Posted by noelex 77

I am sorry, but this is the wrong way around.
If the charger uses current set point to terminate the absorption charge it will terminate later, not earlier with a load.

Suppose the current set point to terminate absorption is say:
Less than 3amps at the absorption voltage.

A three amp load will cause this set point to never be reached. (If its measured at the battery charger not an external shunt). A current based termination of the absorption phase will overcharge not undercharge the battery (unless it is measured at the battery itself)

As you say to help overcome this some battery chargers also (or in some cases only) use a timed absorption phase that will kick in before too much overcharging occurs.

Noelex, chargers use a current set point to terminate absorption mode, thats a common algorithm ( they cant really do much else). BUT, to prevent overcharge they also run a timer to handle the issue where there is load sharing.

The undercharging comes because in a load sharing environment, terminating absorption on a timer results in undercharged batteries.

Dave

[Dockhead]

Originally Posted by goboatingnow

Correct and its a major problem, resulting in undercharged batteries and misleading 'battery charged' lights on chargers.

The charger has only two measurement metrics of concern. Charger current and terminal voltage. Most chargers simply exit absorption mode on a current set point, ( with a overriding safety timer) hence they get confused if supplying a load as well, as usually the timer trips the charger into float mode and the in effect the battery now just trickles charges to completion.

Some chargers also use di/dt as a termination method, but in reality it doesnt do a better job.

IN reality chargers in a load sharing environment, should really have a loads current sensor so that the charger can distinguish between battery load and system load currents.

IN reality battery charging should be done with no load on the battery, in practice not so easily achieved .

Dave

All good theory, but probably doesn't matter much in reality, it seems to me, for most cruisers.

You won't get your batteries to 100% with a genset in any case. It is ludicrous to keep your genset running for hours just to get the last 10%. Or 20%, if you battery charger is very confused by external loads or has a crappy charging algorithm. The difference is pretty academic, IMHO. You've got the same problem whether you have a perfect charging algorithm, or a crappy one, it's just a minor question of degree.

And how does anyone know when you've reached a truly full charge? Your charger doesn't know, and you can't even know for sure without putting a hydrometer on all of your batteries.

All this is washed away when you motor for a longer period of time, or hook up to shore power. And if you live constantly on the hook and don't regularly hook up or motor, then you really need wind or solar anyway, which will do the same thing.


I am actually one of the 5% who often go some weeks without either motoring or hooking up. That's because when not out cruising, my boat lives on a mid-river mooring without shore power. I bought a Rutland wind gen for the finish charging. It didn't work; still trying to sort it out. Meanwhile I use a small Honda generator to put a finish charge on the batts at least once or twice a week. I would never hope for my main charger and main generator to do that job -- they're not suited for it, no matter how finely adjusted. Solar power is pure gold for this purpose, and I'm hoping that if I ever get my wind generator working, I will be able to stop worrying.

When I'm out cruising, I prefer wilder places and avoid marinas as much as possible. I was out for the whole month of July last year, and only spent four or five nights with shore power. I really didn't have any problem at all with power management. If you move every few days, and spend a night or so a week with shore power, your batteries will get a good, full charge quite often enough without much extra effort.

I do sometimes check the batts with a hydrometer and use my Victron charger's forced absorption mode (or equaliziation mode) if I'm unsatisfied with the results, but this is probably overkill even for my use, which is 100x harder on the batts than life on a shore-powered berth.

[noelex 77]

Originally Posted by goboatingnow

Noelex, chargers use a current set point to terminate absorption mode, thats a common algorithm ( they cant really do much else). BUT, to prevent overcharge they also run a timer to handle the issue where there is load sharing.

Dave

Precisely my point. A load will cause the current set point (on its own) to overcharge, not undercharge, the batteries

[goboatingnow]

Originally Posted by noelex 77

Precisely my point the current set point will overcharge, not undercharge the batteries.

Taken alone, placing a charger, that solely terminates absorption on a current set point, and placing that charger in a load sharing environment, MAY, potentially result in overcharging, ( a lot has to do with the exact absorption voltage , the division of load currents etc). However no-one would design a charger with any sort of aggressive absorption mode without a safety timer of some sorts.

Hence the presence of the timer, results in typical undercharging thats very common in load sharing situations.

Dave

[noelex 77]

As has been said the best method of termination of the absorption phase is battery return amps measured at the battery.
Few chargers do this.

Return amps measured at the charger is of very limited value, it will never work with any load.

A adjustable timed absorption phase is next best. Watch the batteries over a few typical cycles a set the average time when the battery return amps are correct.
Some charges add some sophistication using the time in bulk to adjust the absorption time. This can be useful.

The worst option is fixed non adjustable absorption time.

[noelex 77]

Originally Posted by goboatingnow

Taken alone, placing a charger, that solely terminates absorption on a current set point, and placing that charger in a load sharing environment, MAY, potentially result in overcharging, ( a lot has to do with the exact absorption voltage , the division of load currents etc). However no-one would design a charger with any sort of aggressive absorption mode without a safety timer of some sorts.

Hence the presence of the timer, results in typical undercharging thats very common in load sharing situations.

Dave

OK I think we are agreed that a current set point will tend to overcharge the batteries on its own when a load is present.

I am still puzzled why you think a timed absorption phase will always undercharge the batteries with a load.
Even the cheap chargers operate the absorption countdown only when the battery is at the absorption voltage.
Normally the load will have no impact.
If the load is higher than the battery charger can put out and the battery return voltage is not reached it will result in undercharging. In the much more common situation where the load is less than the output of the battery charger, or the voltage drops down to the battery return voltage, it will result in overcharging.

The tendency to overcharge (rather than undercharge) with a load, often occurs because the absorption time is set long as it is seen (incorrectly)only as a safety stop. This is particularly a problem car battery chargers, or marine chargers that have been adapted from car charging algorithms.

This is why an adjustable absorption time is beneficial.