Max BATTRY LOAD LIMITS Whilst CHARGING

[MrMagoo2]

Hi All,

Hoping someone with knowledge can help with a little info.

I seek the limits that can flow directly through as a load on an average agm battery say of say 60 amps while charging.

my specs is 20 a/h constant as a load draw from the battery.

I am asking someone with knowledge of this topic.

IF I HAVE A VERY HIGH AMP CHARGER - say a 70a/h hybrid solar / generator charger and a load of 50 a/h through the battery is this safely doable if my battery is capable of absorbing 20 a/h of current only.

Reading I have done seems to verify that power will flow firstly direct to the load and not to the battery if current is connected to a battery and then only excess towards charging the battery. Thus it should be doable with out frying the battery.

I am not 100% sure on this theory and don't want to experiment as it could be a messy thing to do if it is not correct.

I am asking as I wish to have a 60 a/h 72v battery bank to power my electric motor when moving and docking or low power operation and utilise a high power charger through a generator to power up to 3kw+ when additional engine capacity may be required for longer motoring situations. The generator will be electrically turned on with a relay when battery current draws exceeds 20 amps.

Possible?????????

[GordMay]

There’s no such thing as a battery charger which is "too big" (amps). The batteries will take, the amperage they're going to take (acceptance), and no more, provided that the charging voltage is correct.

[MrMagoo2]

Thanks for your reply.
Can I clarify my question a bit more clear?

I have a battery bank able to handle 20 a/h continuous output with a peak of 55 a/h for a short time. I wish to load this bank up with a load/ draw of around 55 a/h and within 30 seconds start my generator connected to a battery charger of 55 a/h to maintain this draw on a constant on going basis without damaging the battery.......

What are the thoughts on this working - Does any one know or do this????

[sailorboy1]

No such thing as a battery that can “handle 20a/h ...” i assume you mean amps

I dont know why you “want” to do this. But if the battery charger can keep up with the the load plus, the batteries are charging. If you load the system more than the charger can supply you are discharging. It is that simple for your little battery.

[sstuller]

Your units don't make sense. Battery capacity is measured in Amp-hours not amps per hour (amps/hr). A 60 amp - hr battery






will provide 20 amps for three hours or 60 amps for one hour.

Also what is your motor voltage? A 48 volt motor at 20 amps will drain the battery twice as fast as a 24 volt motor at 20 amps.

[DefinitelyMe]

Yes that's fine. If your charger is putting out enough current then you are not loading your battery at all, so yes, it will work fine provided your battery can handle the initial load before the charger comes online.

30 seconds at 55 amps is only 0.46Ah. Although it's a high load it will not draw down your battery capacity by very much at all.

Food for thought: make sure your cabling is sufficient to support the 55A current indefinitely. Depending on the run length (total both positive and negative legs) you're probably looking at somewhere in the 2AWG range to be on the safe side.

[wingssail]

Originally Posted by sailorboy1

No such thing as a battery that can “handle 20a/h ...” i assume you mean amps

I dont know why you “want” to do this. But if the battery charger can keep up with the the load plus, the batteries are charging. If you load the system more than the charger can supply you are discharging. It is that simple for your little battery.

His battery isn't so "little" if it is 20Amps continuously @ 70v. Does that not work out to 1400watts? And if the short term load it can meet is 55amps, by the same thinking it would be equivalent to a CCA of 3850watts.

My meager 450amp hour FLA, at 12v, produces about 200watts continuously (for 24 hours) and a short term load of 1300 watts.

I think I know why he wants to do this, he is thinking of a hybrid design where the generator turns on if the battery load exceeds a certain level. Then he want to know what happens if the generator supplies more amps than the battery can accept in charging mode. FLA? they don't accept more. LI, I don't know, they have very high acceptance rates, don't they?

[BillKny]

Originally Posted by MrMagoo2

Hi All,

Hoping someone with knowledge can help with a little info.

I seek the limits that can flow directly through as a load on an average agm battery say of say 60 amps while charging.

my specs is 20 a/h constant as a load draw from the battery.

I am asking someone with knowledge of this topic.

IF I HAVE A VERY HIGH AMP CHARGER - say a 70a/h hybrid solar / generator charger and a load of 50 a/h through the battery is this safely doable if my battery is capable of absorbing 20 a/h of current only.

Reading I have done seems to verify that power will flow firstly direct to the load and not to the battery if current is connected to a battery and then only excess towards charging the battery. Thus it should be doable with out frying the battery.

I am not 100% sure on this theory and don't want to experiment as it could be a messy thing to do if it is not correct.

I am asking as I wish to have a 60 a/h 72v battery bank to power my electric motor when moving and docking or low power operation and utilise a high power charger through a generator to power up to 3kw+ when additional engine capacity may be required for longer motoring situations. The generator will be electrically turned on with a relay when battery current draws exceeds 20 amps.

Possible?????????

MrMagoo,

You are missing a basic understanding. Putting some hypothetical numbers out there: If the batteries need 10 Amps to meet the current charge needs, and there are loads drawing 50 Amps, the battery charger will supply 60 Amps (if it can). The “extra” 50 Amp do not “go through” the battery. The battery does not even know they are there.

Based on the question you are asking, and the confused set of units you use when discussing currents and charging, I would strongly suggest you have an expert either design your system, or review your design. I know it will cost money, but getting it wrong and spending money on inappropriate equipment, destroying batteries, or starting a fire are all real downsides.

[zstine]

BillKny said it right... If you have a generator with an output capable of the load Amperage draw, you don't need any batteries at all. The generator can just power the load, like a mobile welding setup. If you want to charge batteries too, well that's just another load on the generator. So you have to size the generator to run the load (55amp you said?) plus whatever extra you want to go into the batteries.

[smac999]

if you have a battery that can only provide 20a you must be playing with an RC boat not a real boat...

what kind of junk battery is this.

[wingssail]

Originally Posted by smac999

if you have a battery that can only provide 20a you must be playing with an RC boat not a real boat...

what kind of junk battery is this.

Maybe you might have noticed that it is 20 Amps at 70Volts which is pretty good capacity, from my way of thinking.

[philcun50]

Seems to be a rather adventurous project for one with limited electrical knowledge. There isn’t one simple answer. Kirchhoff’s voltage and current laws would explain this but still you need electrical knowledge to apply them.
Let’s forget the theory and be prepared to cough up the dollars for trial and error or get professional help.
The lovely thing about these posts is they are often misconstrued.
Sounds like fun but hybrid really isn’t suited to situations where there is no regeneration. Hybrid cars are good around town but owners report little advantage on highway runs.

[MrMagoo2]

Originally Posted by DefinitelyMe

Yes that's fine. If your charger is putting out enough current then you are not loading your battery at all, so yes, it will work fine provided your battery can handle the initial load before the charger comes online.

30 seconds at 55 amps is only 0.46Ah. Although it's a high load it will not draw down your battery capacity by very much at all.

Food for thought: make sure your cabling is sufficient to support the 55A current indefinitely. Depending on the run length (total both positive and negative legs) you're probably looking at somewhere in the 2AWG range to be on the safe side.

Thank you - you have directly answered my question as asked. My testing on lower voltage and amperage resulted in this same conclusion.

FYI - Spot on to the post that mentioned the generator coming on to take the high loads as required!

To the doubters I appreciate your efforts any way. To those who dare to try .

This system is for two reasons to boost a small motor for rare times when I need more power and to power my instant hot water for the shower and sink! - Ha this concept I know will get the doubters going!!!!!!!!

[StuM]

My advise, given your obvious lack of any real understanding of how DC electrical systems (chargers/batteries/loads) work is to do some serious reading on the fundamentals (and learn what the basic electrical units are ) before you go near a 72V battery bank!


A good start would be https://www.amazon.com/12-Volt-Bible.../dp/0071392335


(the only difference between 12V and 72V is that one is more likely to destroy your boat or kill you if you mess up).

[MrMagoo2]

Hi Stum,

Thanking you for the advise. Like most forums I take advise with a grain of salt choose what to keep and what to discard based on quality and knowledge of reply's.

Another big thank you to Wingsail and DefinitelMe for your quality knowledgeable and constructive reply's!

Thanking you too strum for your concern for my safety you are right 55 amps at 72 volts is getting deadly and bordering on legal issues here in Australia for a back yarder. - if not handled right peculiarly if one connects the positive lead before the negative! Or if one doesn't ground the generator correctly ie through their body!

Yes I am self taught and well read just could only find one reference of systems out their utilising this concept as a means of maintaining high CONSTANT POWER LOADS THROUGH SMALL BATTERIES - If anyone has first hand experience would love to hear what and how it has gone.....................