Reduce current in a charging circuit

This community is great. I searched through several of the answers and am impressed with how much you guys help.
Here is what I have -
I had a 28v 4200mA DC charger go bad (flooded during a little hurricane called Irma). The company would like around $350 for a replacement.
I own a sdn 5-24-100p which is 28v 5000mA with variable voltage between 23.5-29v. I wired this up and the charging device says 'current too high'. I realize it is 800mA over but was hoping to get lucky. I tried turning the voltage down, attempting to lower current but the end device still did not like this and prompts with same message.

Can I achieve a 28v 4200mA output with a simple resistor? Parallel or series? I think parallel and rated around 25w.

Any help is greatly appreciated.

Hi,

confusing.

* You say you had a charger --- but then you bought a power supply (SDN..) which is different to a charger.

* Then you write "the charging device says 'current too high' " I wonder what this "charging device" is that can speak. ... or "prompt a message".

* Parallel resistor is no good idea.

*****
It seems you don´t know the difference between charger and power supply.
A charger controls the charging current. This charging current depends on battery type (chemistry) charging technology, time, charging status, battery voltage, maybe temperature, age and other parameters.
A power supply usually is made to deliver constant voltage, independent of load current. And it is rated for a maximum current. Usually the user is responsible to keep the load current below the powersupply_rated_current.

It´s not a simple task to create a charger from a power supply.

Klaus

...I had a 28v 4200mA DC charger go bad (flooded during a little hurricane called Irma).

Click to expand...

Perhaps you can open it up, clean the mess and power it up, but that is a different story.

Is that charger for a lead acid battery of 28V? Charging should start from a lower voltage (21V approx) with a current limited to 4A (actually determined by the battery capacity) and the voltage should go up to 29V (approx) and current reduced to a trickle charge value (again that depends on the battery capacity).

Lead acid batteries can stand lots of abuse; you can set the voltage at 28V and limit the current max to 4 or5A (use a series resistor). It should work fine.

My focus was split between posting and a 4 hour conference call earlier. I incorrectly described the issue and will try again -

To elaborate a bit -
Had a 120v AC to 28v DC 4200mA power supply get water damaged. No chance of repair. This supplies power to a downstream lithium ion charger. It is a Husqvarna Automower 430x (robotic mower). Technically it supplies power to the docking station. The Automower has a charging circuit on board.
Is there a reasonable way to modify the output of the SDN power supply (28v DC 5000ma) to the target 28v 4200mA?


As for what prompts a message -

I guess I understand. Your current charger (the SDN power supply) has low impedance and the batteries draw too much current that prompts the relating circuit. With the older charger, as the current was increasing the voltage was dropping.

I still believe that you can use the SDN power supply to connect with the existing charger if you connect a series resistance; say 5W 1E.

Hi,

I had a 28v 4200mA DC charger go bad

Click to expand...

Is it a good idea to recommend a 1Ohms 5W resistor?

It can withstand a current of sqrt(5)A = 2.3A ...while the original one is rated for 4.2A.
4.2A causes a power dissipation of almost 18W.

I must agree there is a small chance that this solution may work...but for sure not with almost equal performance.
There is a chance that the resistor will overheat and burn. Protect nearby devices from catching fire.

I personally can't recommend any solution without knowing how the original circuit worked.

Klaus

@cmitra. Is the high current draw on psu because the battery is low. And as battery builds up the current will reduce in the same way a battery charger does. Or will the psu deliver that high current irregardless of the charge on battery.

- - - Updated - - -

Never mind, skimmed through post 5 to quickly.

Solution - purchased a 30v 5a variable power supply and dialed it in as required. I did not see a clear way to get the SDN to function in the desired way. Still open to suggestions if there is one.

I think if you will connect a 800mA load in series with the charger then it will drop the extra current and charge without any errors.

How about you add a current regulator/limiter between your load and the 5A supply?

- - - Updated - - -

Also, that 5000mA should be the maximum limit, not a regulated current we should operate on. Your battery manager should automatically limit the current. Lithium ion battery charging is more sensitive and complex compared to vehicle batteries.

You can also put a current limiter/regulator between power supply and charger. Be sure voltage input is higher than needed because the regulator/limited will have to drop some voltage.

This supply doesn't sound like a useful charger input voltage...
Fold Forward (Current rises, voltage drops to maintain constant power during overload up to max peak current)
Makes me think the supply is overloaded so it's supplying it's max peak current probably at some reduced voltage (instead of 28V).

More likely the manufacturer uses a current limited supply to check for non-Husquvarna made power supplies, forcing you to spend the $350 on their supply.

ads-ee said:

This supply doesn't sound like a useful charger input voltage...
Fold Forward (Current rises, voltage drops to maintain constant power during overload up to max peak current)
Makes me think the supply is overloaded so it's supplying it's max peak current probably at some reduced voltage (instead of 28V).

Click to expand...

More likely it is operating undervoltage, thus compensating the power through higher current. However, it's weird that it didn't say other error than overcurrent. Such complex and sensitive battery type should have at least those features.