# How to calculate resistor and capacitor values for low pass filter for battery charging / dischargin

#### Oz V

Joined Sep 7, 2019
4
i want add a Low pass filter between battery and 720watt ups inverter. because dc charging wires has some ripple voltage. I want to make it pure dc... charging voltage is max. 14.4vdc and charging amps between 10-20amps... and on inverter mode the same wires draw 60 amps at 10-12.6vdc approx 680watts to 720watts. my question is how to select Resistor value and capacitor value for this low pass filter?

#### AlbertHall

Joined Jun 4, 2014
9,671
What problem is the ripple causing?
The battery won't care.

If you add a resistor between the charger and the battery it will likely mess up the charger's sensing of the battery voltage.

#### TeeKay6

Joined Apr 20, 2019
572
i want add a Low pass filter between battery and 720watt ups inverter. because dc charging wires has some ripple voltage. I want to make it pure dc... charging voltage is max. 14.4vdc and charging amps between 10-20amps... and on inverter mode the same wires draw 60 amps at 10-12.6vdc approx 680watts to 720watts. my question is how to select Resistor value and capacitor value for this low pass filter?
@you need to differentiate ripple measured from end to end of a length of a wire--a problem possibly solvable by using heavier wire--and ripple measured at some output or input ( e.g. between ground and power wires ) of/at some circuit block (e.g. output at battery, input of inverter, output of charger, input at battery), a more difficult problem to solve...if a solution would actually bring some benefit. Adding any filter that uses resistance will necessarily involve power loss...possibly much more power loss than the ripple causes. First step, determine exactly what ripple you see and where in the circuit you see it. Second step, decide whether there is any reason to try to reduce the ripple. Third step, knowing exactly where the ripple is measured and how much it is (preferably via a scope trace)--and having decided that it must be reduced--then decide on a suitable approach for reducing the ripple. This might be simple or very complex. At the current levels you mention, the cost to reduce ripple may be significant. In any case, to proceed without scope traces of the ripple would be foolish.

#### Oz V

Joined Sep 7, 2019
4
@you need to differentiate ripple measured from end to end of a length of a wire--a problem possibly solvable by using heavier wire--and ripple measured at some output or input ( e.g. between ground and power wires ) of/at some circuit block (e.g. output at battery, input of inverter, output of charger, input at battery), a more difficult problem to solve...if a solution would actually bring some benefit. Adding any filter that uses resistance will necessarily involve power loss...possibly much more power loss than the ripple causes. First step, determine exactly what ripple you see and where in the circuit you see it. Second step, decide whether there is any reason to try to reduce the ripple. Third step, knowing exactly where the ripple is measured and how much it is (preferably via a scope trace)--and having decided that it must be reduced--then decide on a suitable approach for reducing the ripple. This might be simple or very complex. At the current levels you mention, the cost to reduce ripple may be significant. In any case, to proceed without scope traces of the ripple would be foolish.
Can i only use Shunt Capacitor of 1 farad to cutoff higher frequencies. battery's internal resistance is 0.2ohm . if i use 1 farad capacitor. can it cuts frequencies above 0.7 hz.

What problem is the ripple causing?
The battery won't care.

If you add a resistor between the charger and the battery it will likely mess up the charger's sensing of the battery voltage.
Ripple Causing shock when touching the wet battery when on charging mode.. and battery is glowing ac light bulb on charging mode.... the charging wires come from some transformer like stuff may be i think dc chopper ... on plus side there is a diode for charging mode... and a capacitor and a resistor parallel to that diode.

when this ups inverter was operating on 1 phase utility meter... it was functioning right... but now i operate it on single phase from 3 phase meter...

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#### TeeKay6

Joined Apr 20, 2019
572
when this ups inverter was operating on 1 phase utility meter... it was functioning right... but now i operate it on single phase from 3 phase meter...
Ripple is very unlikely to be the source of a shock that you feel. From what info you have given, my guess is that when you switched from 1-phase power to one phase of 3-phase power you lost a ground connection.

#### Oz V

Joined Sep 7, 2019
4
Ripple is very unlikely to be the source of a shock that you feel. From what info you have given, my guess is that when you switched from 1-phase power to one phase of 3-phase power you lost a ground connection.
Maybe! but it has ground connected with its own casing... by the way, if i connect a 1 farad capacitor between battery and ups inverter... then what will happen? any improvements?

#### Ylli

Joined Nov 13, 2015
933
but now i operate it on single phase from 3 phase meter...
That can be your problem. You may be operating the whole unit above ground. could be very dangerous. You need to get it Properly connected back on the single phase service, or you may need to add an isolation transformer.