What is maximum charge current into lead acid battery?

Hello,
What is the maximum current that you can charge a 5A.hr, 12V lead acid battery with?
Unfortunately, they wont give the battery datasheet out.
They have asked us to put the battery across the output rails of a 14.4V, 300W Full bridge SMPS, and said that we should just let the battery charge from that. They said we should just make the Full bridge output current limit at 20 Amps. (ie, let the output voltage fall to whatever it falls to as long as it gives a maximum of 20 Amps out)
However, we are worried that allowing 20 Amps to flow into a 5A.h Lead acid battery may overheat it.

We offered them to use a charger, from the full bridge output to the battery, but they said don’t bother.
The battery is the starter battery of an Electric car. (ie the one that switches the contactors that connect the high voltage battery to the motor to make it start)

Hi,

If you are responsible for function and safety you should insist on getting the datasheet.

Otherwise you risk explosion that may hurt people.

Klaus

This may actually be o/k, but only if its a flooded wet cell, not a sealed paste type gell battery.

Charging methods like this are pretty universal in all automotive applications. The engine alternator is set to something like 14.2 volts, and the maximum current limited by the alternator to 30, 50, 75, 100 amps (or something) for what might be only a 40 Ah flooded battery.

What happens is, as soon as the engine fires, the peak battery current will flash right up to the current limit value, than after only perhaps a minute begin to very quickly fall away to a much lower charging current as the battery terminal voltage reaches the set 14.2 volts.

Its pretty brutal, but because the maximum peak charging current is only there for a very brief time, overheating and excessive gassing are not a problem. Its a proven system, but...

The battery should be the flooded and vented automotive type, and be rated as an "engine starting battery". I have never seen a lead acid battery as small as 5Ah battery rated for engine starting.
If its a sealed lead acid battery, the life is going to be incredibly short because its going to vent and very quickly dry out, and as Klaus says, it may go *BANG*.

treez said:

Hello,
What is the maximum current that you can charge a 5A.hr, 12V lead acid battery with?

Click to expand...

you may be aware that the 5Ahr battery charging current is 0.5A.
How 20A is arrived at for a 5AHr battery?
Is there any new method available ?

thanks, BTW, there is no alternator here, it is totally electric vehicle. Since this battery only has to switch some contactors on, it seems a bit strange that it should be such a high current rated battery. -Unless it needs to be so big so it doesn't self discharge quickly.

I’ve been told that if this lead acid starter battery is flat at first, (eg 9V) then the full bridge output will get pulled down to that level (9V). But then wont this affect the lights and other stuff, which would be expecting that rail to be 12-13.8v?

I realise there is no alternator, but just pointing out that a current limited source at say 75 amps does not mean the battery is going to charge at 75 amps for more than a very brief initial period, maybe only seconds. Same with your 20 amp switcher.

If the battery is totally flat at 9v, connecting a 14.4v charger will instantly raise the voltage to 14.4.
There may initially be very little charging current if the battery internal resistance is quite high, but it should slowly increase, then decrease as it gains charge.
It may never see the full twenty amps.

If its a sealed batter, as I suspect it is, I would get some data about recommended charging voltages and currents.
If the manufacturer will not supply that, find a better manufacturer with a different battery that will.

Lithium technology might suit you better for this application.
It costs more, but it has far lower size and weight, and it will recharge much faster, because it will stay at current limit right up to full charge, where you then disconnect the charger.

A 5Ah lithium can be charged at 5A and it will be guaranteed full in one hour from total discharged condition.
A lead acid battery will have a continually reducing charging current over hours and days, never actually reaching a full 100%

Unfortunately, they wont give the battery datasheet out.

Click to expand...

By any chance, could 'they' have a new type of battery (perhaps non-commercialized technology), which they would like to introduce in electric vehicles, but they need to know what is or is not possible toward making it work? So they make up things such as 'We'll just connect it across a 14V supply and send 20A through it'... Because their real aim is to find out what experts have to say about it?

Could there be a battery so advanced that it can stand up to such severe treatment, with no disasters? (Fuel cells, perhaps? There are occasional hopeful news reports about developing a battery technology which has extremely high energy density.)

Could 'they' be a government agency? Not sure of what they're doing? So they 'pump' you for information by bringing up a lot of hypotheticals, without giving you specs or any real information?

thanks, incidentally do you know what a starter battery for an EV has to do? , in total, is it just switching the contactors?

Power up the low voltage motor drive circuits for the invertors, its unlikely they will be running from 400V! Operate the electric brakes, lights, horn, radio. . . .?
Perhaps its the long term start reservoir. Suppose you have all your miscellaneous circuits of one battery, such as the clock and burglar alarm then after a month or so this battery would be flat, so you would need a jump start in the airports longterm carpark. Perhaps this battery has no other use, so it would remain at 100% charge for a long time.
Frank

Hi,

you may find a lot of usefull informations in the internet.

like this one:


Don´t rely on hobbyists experience, rather rely on large companies (battery manufacturers) informations.

Klaus

i read somewhere it is from c/10 to maximum of c/5 ,
but if your battery is flooded lead acid , as you increase the charging current , gassing will increase
you have also to take care of battery compartment temperature because it will affect the charging cycle

I understand that the intended battery operation is similar to constant I/constant V charging by a car alternator. As said, it has to be presumed that the battery is suited for the possible high initial charging current. 5Ah is typically a motorcycle battery, respective current limit would be in a few amperes range. In so far there's a certain battery/charger mismatch in your design. It can basically work, but battery lifetime is most likely reduced.

It should be also noted, that lead acid battery lifetime is an a range of hundred full cycles only, every complete discharge is reducing it by a noticeable amount and storing the battery in discharged state quickly exhausts it.

Usage of a SMPS as battery charger demands constant I/constant V operation which isn't provided by all SMPS. Industry standard SMPS implement different overcurrent protection methods, e.g. hickup current limiting, which is completely unsuited for a battery charger.

Power up the low voltage motor drive circuits for the invertors, its unlikely they will be running from 400V! Operate the electric brakes, lights, horn, radio. . . .?

Click to expand...

Thanks, this was in reply to my question about what the low voltage starter battery actually does in an electric car. (not the main power train battery)
I believe that the starter battery merely switches the contactor which connects the High voltage vehicle battery to the vehicle...from there, a full bridge converter provides the low voltage rail for the lights , radio etc, so there is no need for this low voltage start battery to do anything else other than switch on the contactor of the high voltage vehicle battery.

So I cannot understand why this low voltage starter battery is obviously big enough to be able to take 20 amps of current (even if only for a short time)...surely, the low voltage starter battery should just be say a tiny rechargeable 0.1Ah lithium cell, as all it has to do is switch the contactor?

And surely it should have its own small sepic charger...not be slammed across a 14V, 300W voltage rail?

" Perhaps this battery has no other use, so it would remain at 100% charge for a long time." Would your .1 Ah lithium battery be good after a month? For a car starter, the solenoid which is switching 12V @200 A needs 12V @50 A(two step action, first step 10A, second 50A), so a current gain of only 4 or so. This due to the high force spring needed to separate the contacts fast on opening, else the DC arc will burn the contact. So perhaps your starter relay needs more then 20 amps, hence the "powerful" battery?
Frank

If you already have a 400v to 14v at 20A switching power supply, and all the loads are quite small, why is a low voltage battery needed at all ?

because the 400v to 12v, 20A supply is sourced by the high voltage vehicle battery, and we must switch that out when the car isn't used...so then we need the starter battery to switch it back in again when we again want to use the car..

I think you can get tiny lithium batteries which last for ages, with very little self discharge.

I cant see the contactor needing that much actuation current...when it switches on, there is then a fet switch, so there is no current flow across the contactor terminals at the point when it is switched......then after is is contacted, the fet switch closes and then an inrush limiter.

I'm thinking about the various loads this battery has to power. Won't it be needed to run the radio, and windows, and stuff plugged into the cigarette lighter?

And suppose the primary battery bank gets depleted after powering the grid. Then won't this little battery be crucial, to power the gps so the tow truck can get a precise fix on the car? Won't this little battery be needed to recharge the cell phone so the owner can call a tow truck?

I suspect this battery needs to be rechargeable in a short time, in case a child has played with the windows while Mom is in the mall. Brake lights, stereo, horn. Etc.

Therefore it will need some special consideration. Just a notion gathering in my mind, influenced by recent threads regarding electric vehicles.

Even if the big 400v traction battery completely dies and has only 100v, a decent switching power supply should still be able to keep up the 14.4v to power your bad ass sound system.

There's much discussion in this thread about how the power supply of an electric car could be designed.

The original question is however about designing a charger for an existing system. Suggesting a different battery type is probably beyond the scope of this design, modifying the HV wiring surely is.

thanks, just found out that this battery could be the battery in a petrol/electric vehicle, as well as in a pure electric vehicle.