Battery charging from a generator. Voltages and Current Discussion.

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Mark Payne

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Hi All

I am sanity checking myself here and would welcome comments and discussion:

Since killing off a 5 year old set of AGM batteries by accidental over-discharge I am being more careful and attentive with my new Gel cells. I would rather not kill off another £1000.

I have installed a BMV712 Victron battery monitor and am better able to monitor SOC (State Of Charge %), battery voltage and charge/discharge currents.
My shore charger is a Mastervolt Mass Combi 12 2200/100 with a 100A charge capability.
My batteries are 3 * 130AH Gel ... They are branded newmax from Seatronic (Click here)

My reading suggests that maximum charge current during bulk/absorption phase would be 0.25 * C20 discharge rating (130AH) so that would be around 33A per battery... say 100A for the bank.
That makes sense with the maximum charge capability of the Mastervolt shore charger. I am all good with what the Mastervolt is up to and I understand the 3 phase battery cycle Bulk - Absorption - Float.

My question is about what happens when I charge from my Fischer Panda 4000 generator. I am currently sat in Salcombe with it charging bringing my batteries back from a 65% SOC back up to 90% ... I don't bother going more than the 90% on the generator as that just takes too long. I will wait for shore power to do that.

I have noticed when I first start the generator, the current delivered at the beginning is very high. I have seen 180A flowing into the bank ... that's 60A per cell.
This would appear to be more than the "recommended" 0.25*C20 Capacity of the batteries (around 30A per battery). Should I be worried about this?

What the generator is doing is holding constant voltage across the bank of around 13.6 V at the start of the charge when I am hitting the peak 180A charge.
30 mins in and I am up into the region 80% SOC, current has fallen to 80A (26A per battery) with 13.9V across the bank.

These voltages themselves are not that high and closer to what the final Float voltage would be.
But the current capability of the Panda worries me as it hits the batteries at the start.
Should I be worried?

Cheers
Mark
 
Mark Payne said:
Hi All

I am sanity checking myself here and would welcome comments and discussion:

Since killing off a 5 year old set of AGM batteries by accidental over-discharge I am being more careful and attentive with my new Gel cells. I would rather not kill off another £1000.

I have installed a BMV712 Victron battery monitor and am better able to monitor SOC (State Of Charge %), battery voltage and charge/discharge currents.
My shore charger is a Mastervolt Mass Combi 12 2200/100 with a 100A charge capability.
My batteries are 3 * 130AH Gel ... They are branded newmax from Seatronic (Click here)

My reading suggests that maximum charge current during bulk/absorption phase would be 0.25 * C20 discharge rating (130AH) so that would be around 33A per battery... say 100A for the bank.
That makes sense with the maximum charge capability of the Mastervolt shore charger. I am all good with what the Mastervolt is up to and I understand the 3 phase battery cycle Bulk - Absorption - Float.

My question is about what happens when I charge from my Fischer Panda 4000 generator. I am currently sat in Salcombe with it charging bringing my batteries back from a 65% SOC back up to 90% ... I don't bother going more than the 90% on the generator as that just takes too long. I will wait for shore power to do that.

I have noticed when I first start the generator, the current delivered at the beginning is very high. I have seen 180A flowing into the bank ... that's 60A per cell.
This would appear to be more than the "recommended" 0.25*C20 Capacity of the batteries (around 30A per battery). Should I be worried about this?

What the generator is doing is holding constant voltage across the bank of around 13.6 V at the start of the charge when I am hitting the peak 180A charge.
30 mins in and I am up into the region 80% SOC, current has fallen to 80A (26A per battery) with 13.9V across the bank.

These voltages themselves are not that high and closer to what the final Float voltage would be.
But the current capability of the Panda worries me as it hits the batteries at the start.
Should I be worried?

Cheers
Mark
Click to expand...
I don't understand the 60A per cell. Presumably you mean 60A per battery? That will be no problem for 130Ah batteries but will soon reduce as the batteries self-regulate. Nothing to worry about.

Richard
 
Hi Richard

Yes indeed I meant 60A per battery. My typo, sorry. Of course technically it amounts to the same thing ;-) Thanks for the reasurance.

This is kind of what I felt as the current flow does indeed regulate down but when I saw that 180A it made
me go woah! Thats some serious generator capability. The genny is way more capable than the alternator or shore batt charger. I guess thats what 3KW gets you at 12V
 
What does your engine alternator pump out when battery is low SOC ?

Many people fit higher output alternators ... some as much as 100A rated or more. Even a bog standard car based alternator on a boat engine is often 45 - 60A rated ...

Lets say you start your engine from a single battery ... and its cold .. takes a few turns over and battery SoC drops .. engine starts .. revs pick up and alternator kicks in .... you could easily be pumping in high ampage till battery picks up.
 
confused, is the Fisher Panda capable of charging batteries as is, or simply provides the 230V to the Mastervolt to do it's thing?
 
vas said:
confused, is the Fisher Panda capable of charging batteries as is, or simply provides the 230V to the Mastervolt to do it's thing?
Click to expand...

I'm also confused. I thought the Panda 4000 was AC only.

Edit: It seems there's a 12v output version called the Panda PMS AGT 4000.
 
pvb said:
I'm also confused. I thought the Panda 4000 was AC only.

Edit: It seems there's a 12v output version called the Panda PMS AGT 4000.
Click to expand...
Oh, ok never heard of such a thing,
However is there any point in such a thing? I guess it does have AC output, shouldn't the OP use that to charge his batteries instead?
 
If its an AGT Fisher panda then the out put is what you get you need enough batteries to absorb the peak. The older controllers aren't all that sophisticated.

If your a 4000i then your supplying AC to the mastervolt combo unit, then this can be programmed via the master bus. You can then limit the maximum charge current and customise the charging profile for your battery type.

If you can change out the victron battery monitor for the mastervolt version then the system is a lot more integrated and works well and can even be displayed on your plotters without to much fuss.

For those who don't know the AGT series of DC generators have been fitted to RNLI lifeboats for years and are more efficient than running your main engine for battery charging (especially larger main engines) they also great for hybrid power and propulsion systems.

Check out polar power in the states if you want to learn more about dc generators for marine use (probably the best of the breed)

No affiliation with either companies!
 
My Panda is 12v dc output. It charges the batteries and the mastervolt makes the mains. The point is that it is harmful to the 54HP Yanmar to just run it at anchor for long periods to recharge. The engine is not loaded this way and it will not run correctly and over time that does not do it any good.

This is the point of a smaller genset.
Appropriate sized engine to do job. quiet and fuel efficient.

Yes I agree.. it is strange is it not that this works at the 12v level but this is indeed the system I inherited. This is a Najad 380 and the instalation is typical of the time (2006).

Sorry I was away from the thread for a while. Been sailing!
 
Mark Payne said:
Yes I agree.. it is strange is it not that this works at the 12v level but this is indeed the system I inherited. This is a Najad 380 and the instalation is typical of the time (2006).
Click to expand...

Actually ... on balance. I like the 12V independant nature of the genset being able to charge my house systems without the Mastervolt. 2 seasons ago my original Mass Combi let go with a nasty noise a some smoke. I was able to isso it and still have a method of charging without using the main engine. That is an upside as it eliminates a single point of failure.
 
jwfrary said:
If its an AGT Fisher panda then the out put is what you get you need enough batteries to absorb the peak. The older controllers aren't all that sophisticated.

If your a 4000i then your supplying AC to the mastervolt combo unit, then this can be programmed via the master bus. You can then limit the maximum charge current and customise the charging profile for your battery type.

If you can change out the victron battery monitor for the mastervolt version then the system is a lot more integrated and works well and can even be displayed on your plotters without to much fuss.

For those who don't know the AGT series of DC generators have been fitted to RNLI lifeboats for years and are more efficient than running your main engine for battery charging (especially larger main engines) they also great for hybrid power and propulsion systems.

Check out polar power in the states if you want to learn more about dc generators for marine use (probably
Click to expand...

Nice tips here thank-you. Yes its an AGT Panda circa 2005/2006.

Your input again has me asking the question... should I be hitting 130AH Gel batteries with 60A at the start when it looks like 30A max charge rate is more recomended.

Yes the Panda AGT is NOT very sophisticated agreeed!
 
Mark Payne said:
Nice tips here thank-you. Yes its an AGT Panda circa 2005/2006.

Your input again has me asking the question... should I be hitting 130AH Gel batteries with 60A at the start when it looks like 30A max charge rate is more recomended.

Yes the Panda AGT is NOT very sophisticated agreeed!
Click to expand...

Batterys only get high rates when are low state of charge - so the 60A indicates you have pushed batterys down to a pretty low level ...
I would suspect that quite quickly the rate drops as the battery charge tops up ... your early post says :

"What the generator is doing is holding constant voltage across the bank of around 13.6 V at the start of the charge when I am hitting the peak 180A charge.
30 mins in and I am up into the region 80% SOC, current has fallen to 80A (26A per battery) with 13.9V across the bank."

The Voltage may be constant for short period at start - but it seems from your numbers that the voltage and amps are not constant ...
CV and CA are specific terms in this ... Constant Voltage where the amps change as charge level increases ... Constant Amps when voltage changes as charge level increases.

I know some will argue about this - but you can choose chargers of either type CA or CV (depends on use intended) or more usual where the charge routine changes from one for bulk charge to the other for final charge top-off.

But back to your 60A ... if it was me ? Id go with it ... but that's me !!
 
So the
Mark Payne said:
Nice tips here thank-you. Yes its an AGT Panda circa 2005/2006.

Your input again has me asking the question... should I be hitting 130AH Gel batteries with 60A at the start when it looks like 30A max charge rate is more recomended.

Yes the Panda AGT is NOT very sophisticated agreeed!
Click to expand...
Well an AGT 4000 is capable of Circa 300 amps peak and I think a battery capacity of around 1200 amps is recommended.

Im not super familiar with the control board from that era but as far as I know its not that and quite crude ( they have only just switched the agt to can bus)

I expect this fast charging combined with deep cycling and sulphation from mit going though to absorbtion is whats limiting your battery lifespan.

Lead acids of whatever technology aren't Actully particularly good at the job we are asking them to do on a yacht! starting engines they are really good at! Deep cycling not so much and fast charging even less so!

Your agt would perform much better with a lithium bank but obviously there's a cost there and this is something we have to weigh up, accept a banks short life span or invest in the right technology for the application.

Prob only need to change the bank a couple of times to pay for the lithium investment, that and you'll save quite a bit of diesel and generator time too!
 
To limit the max charge current of the generator you only need add some resistance to the what must be a massive wire carrying the charge current. So fit a thinner longer wire in pos and or negative or even fit an amp meter shunt. Just a few fractions of an ohm will drop the voltage at 180 amps. Slight volt drop will reduce current.
So drop one volt at 180 amps will need 1/180 ohm. That one volt drop will reduce charge current. Perhaps .5 volt drop will do. The point is at 90 amps charge current only half of original volt drop so less concern. The outcome will be a slightly longer charge time. You could experiment making a resistor out of a piece of stainless steel strap say 20 gauge one inch wide and 3 inches between attachment bolts. Just a thought ol'will
 
William_H said:
To limit the max charge current of the generator you only need add some resistance to the what must be a massive wire carrying the charge current. So fit a thinner longer wire in pos and or negative or even fit an amp meter shunt. Just a few fractions of an ohm will drop the voltage at 180 amps. Slight volt drop will reduce current.
So drop one volt at 180 amps will need 1/180 ohm. That one volt drop will reduce charge current. Perhaps .5 volt drop will do. The point is at 90 amps charge current only half of original volt drop so less concern. The outcome will be a slightly longer charge time. You could experiment making a resistor out of a piece of stainless steel strap say 20 gauge one inch wide and 3 inches between attachment bolts. Just a thought ol'will
Click to expand...
Hi William.
Unfortunately I think this is dangerous thinking. We run the risk of burning the boat down with this approach!

1. The generator is a closed loop controlled supply for voltage (within its power capability) so we regard it as having zero drive impedance.

2. Lets assume the cell bank can absorb 200A at 13.8v across. That means the bank has an in circuit impedance of 13.8/200 = 0.07 Ohms.

3 If we wanted to half the supply current to say 100A we would have to double the circuit impedance to 0.14 ohms by adding 0.07 ohms of resistance (a thin wire or shunt or similar was your suggestion).

The power dumped on the device is I^2 *R (I squared R) = 100*100*0.07 = 700 watts... thats nearly 1KW of heat which WILL burn the boat down.

So this should not be tried at home folks!

4 Just to say that a 300A current measurement shunt typically is calibrated to drop 75 mV (0.075V) at 300A. That is an in circuit impedance of 0.075/300 = 0.00025 ohms. Such a shunt would be a meaningless change to the overall circuit impedance which is indeed the point of it (measurement without change). Only 22 watts of power is warming up this shunt at full rated current.

Best

Mark
 
Mark Payne said:
Hi William.
Unfortunately I think this is dangerous thinking. We run the risk of burning the boat down with this approach!
1. The generator is a closed loop controlled supply for voltage (within its power capability) so we regard it as having zero drive impedance.
2. Lets assume the cell bank can absorb 200A at 13.8v across. That means the bank has an in circuit impedance of 13.8/200 = 0.07 Ohms.
3 If we wanted to half the supply current to say 100A we would have to double the circuit impedance to 0.14 ohms by adding 0.07 ohms of resistance (a thin wire or shunt or similar was your suggestion).
The power dumped on the device is I^2 *R (I squared R) = 100*100*0.07 = 700 watts... thats nearly 1KW of heat which WILL burn the boat down.
So this should not be tried at home folks!
4 Just to say that a 300A current measurement shunt typically is calibrated to drop 75 mV (0.075V) at 300A. That is an in circuit impedance of 0.075/300 = 0.00025 ohms. Such a shunt would be a meaningless change to the overall circuit impedance which is indeed the point of it (measurement without change). Only 22 watts of power is warming up this shunt at full rated current.
Best
Mark
Click to expand...
You cannot regard the battery has a resistance that obeys ohms law

You are suggesting adding 0.07 ohms to the circuit in order to reduce the current to approx 100 amps. If this was the result your added resistance would have a potential difference of 7 volts across it ( 100 x 0.07) reducing the volts applied to the battery to 6.8 volts at which point there would be no charging . Clearly this line of reasoning is nonsense

You probably only need to reduce the volts by about 0.1 volt or so in order to reduce the current to 100 amps , This would result in a power dissipation of only 10 watts Even if it required a volts drop of 0.2 volts the power dissipation will only be 20 watts at 100 amps
 
I need to think more on that!
VicS said:
You cannot regard the battery has a resistance that obeys ohms law

You are suggesting adding 0.07 ohms to the circuit in order to reduce the current to approx 100 amps. If this was the result your added resistance would have a potential difference of 7 volts across it ( 100 x 0.07) reducing the volts applied to the battery to 6.8 volts at which point there would be no charging . Clearly this line of reasoning is nonsense

You probably only need to reduce the volts by about 0.1 volt or so in order to reduce the current to 100 amps , This would result in a power dissipation of only 10 watts Even if it required a volts drop of 0.2 volts the power dissipation will only be 20 watts at 100 amps
Click to expand...
Yes your potential divider argument makes sense and unpicks my points above.

However I am still not convinced about the 0.1 V reduction argument (making a big reduction in current) as with a high power genset able to hold a stiff controlled voltage rail (albeit 0.1v lower) ... the initial current delivery to the lower SOC bank is still going to be very high? Possibly unaltered?

Gut feel. I cant prove that. My last attempt at figuring that out was clearly suspect.
 
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