# What should be the output voltage of a step down transformer for a 10amp car battery charger

#### B Bobby

Joined Oct 6, 2018
8
I was looking to create a 10amp manual charger with variable voltage and current option for flooded lead acid battery (normally used in cars).

I only know some basic electronics and usually make solutions by integrating off the shelf modules.

For this project I have to first buy or order a custom built 10amp step down transformer but I was bit confused what should be the secondary output voltage for this transformer

To charge a FLA battery we usually need up to 14.2V and to equalize we need 15V that is maximum output we need from a battery charger

Therefore my question is should I look for a transformer with exactly 15V secondary output? or higher than this and why?

Thanks,

#### crutschow

Joined Mar 14, 2008
25,270
Depends upon the voltage drop you current/voltage control circuit will require.

And the DC voltage from a rectifier-filter is about 1.4 times the RMS AC transformer voltage, so for 15Vdc you would need at least an AC output of 15V/1.4 = 10.7Vrms from the transformer.
Typically you would likely use a transformer with a standard output of 12.6Vac.

A custom-built transformer for this would be rather expensive.

#### B Bobby

Joined Oct 6, 2018
8
Depends upon the voltage drop you current/voltage control circuit will require.

And the DC voltage from a rectifier-filter is about 1.4 times the RMS AC transformer voltage, so for 15Vdc you would need at least an AC output of 15V/1.4 = 10.7Vrms from the transformer.
Typically you would likely use a transformer with a standard output of 12.6Vac.

A custom-built transformer for this would be rather expensive.
Many thanks for this valuable information

#### ian field

Joined Oct 27, 2012
6,539
I was looking to create a 10amp manual charger with variable voltage and current option for flooded lead acid battery (normally used in cars).

I only know some basic electronics and usually make solutions by integrating off the shelf modules.

For this project I have to first buy or order a custom built 10amp step down transformer but I was bit confused what should be the secondary output voltage for this transformer

To charge a FLA battery we usually need up to 14.2V and to equalize we need 15V that is maximum output we need from a battery charger

Therefore my question is should I look for a transformer with exactly 15V secondary output? or higher than this and why?

Thanks,
A lot of basic chargers had "impedance wound" transformers that intentionally have lousy regulation. the electrolyte in a flat battery is not far off pure distilled water and not easy to get any charging current on the move. it can take around 30V to get going if a flat battery is showing first signs of sulphation - maintaining that voltage after it starts drawing current wouldn't end well!.

A linear electronic charger suggested in yuasa literature starts with constant current at 1/3 of Ah rating and 29V capability. The voltage will be pulled down as charging current picks up. a comparator detects 10.8v and latches a flip flop that switches over to constant voltage charging. You should probably aim for about 14.4V - that will gas the distilled water away after a while. Smart chargers usually drop back to about 13.6V. A good compromise is about 13.8V, but you still can't leave it float charging for months.

#### B Bobby

Joined Oct 6, 2018
8
A lot of basic chargers had "impedance wound" transformers that intentionally have lousy regulation. the electrolyte in a flat battery is not far off pure distilled water and not easy to get any charging current on the move. it can take around 30V to get going if a flat battery is showing first signs of sulphation - maintaining that voltage after it starts drawing current wouldn't end well!.

A linear electronic charger suggested in yuasa literature starts with constant current at 1/3 of Ah rating and 29V capability. The voltage will be pulled down as charging current picks up. a comparator detects 10.8v and latches a flip flop that switches over to constant voltage charging. You should probably aim for about 14.4V - that will gas the distilled water away after a while. Smart chargers usually drop back to about 13.6V. A good compromise is about 13.8V, but you still can't leave it float charging for months.
Thanks for this great info,

Does this mean I should have a power supply with 29V voltage output but of course voltage cutoff will be happened at 14.2V or 14.4V?

Actually I wanted to have this charger only for occasional battery maintenance. The 3 or n-stage automatic chargers sometimes refuse to charge a battery which somehow went below 10v or 9V.

Therefore my idea was to just have a power supply with a voltage cutoff module to prevent any overcharge, other than this no other complex charging routine using micro-controllers

#### ian field

Joined Oct 27, 2012
6,539
Thanks for this great info,

Does this mean I should have a power supply with 29V voltage output but of course voltage cutoff will be happened at 14.2V or 14.4V?

Actually I wanted to have this charger only for occasional battery maintenance. The 3 or n-stage automatic chargers sometimes refuse to charge a battery which somehow went below 10v or 9V.

Therefore my idea was to just have a power supply with a voltage cutoff module to prevent any overcharge, other than this no other complex charging routine using micro-controllers
you don't seem to have taken much notice of what I said - charger transformers deliberately have lousy regulation so the off load voltage is high enough to get a flat battery to take charging current. A "stiff" transformer that can get it going, will buckle the plates when it starts drawing current.

I'd probably use a general purpose equipment transformer that gives about 30V after rectification - a current limit circuit is then REQUIRED.

Its just so much easier to nip in a discount store and get a semi-smart fast charger - the smart or "maintenance" types are usually very slow.