Transformer for a car battery charger

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,350
Hi guys! I am trying to use a transformer for making a car battery charger with LM2596. I have decided to charge at 0.5A(I know its slow) and I am thinking this transformer would be good:

50VA 230, 380, 400, 415V / 24V, 2,1A

1588953872393.png
1588953899113.png
 
Last edited by a moderator:

nsaspook

Joined Aug 27, 2009
7,296
No, my battery just needs a charger.
What size battery. A 'normal' car battery needs a lot more current than 0.5A usually to recharge.
0.5A will replace standby losses (self-discharge during storage) in a fully charged battery but it won't have the energy needed to recharge a dead one.
 

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,350
What is the discharge rate per day? It should be less than 1% according to the standard, but in reality I see it to be a lot more.
 

nsaspook

Joined Aug 27, 2009
7,296
What is the discharge rate per day? It should be less than 1% according to the standard, but in reality I see it to be a lot more.
>1% for lead-acid? Sure for a new, top condition battery. Preventing sulfation is another important consideration. You need to blast those plates with extra energy to prevent crystal build-up.

It's like climbing a steep hill. Going up to the top (regular battery charger) is much harder than circling (trickle charger) the hill, maintaining the same height.

 
Float charging a car battery takes at least 200mA from my measurements, depending on the age of the battery, purity of water etc. so I think there's no point to making a 500mA charger, especially with a big 50VA transformer.
OP could just use a linear regulator by undervolting the primary (it has 4 taps) to give around 17VDC raw. I'd just use an LM317 for 1A. The low cost 2A float chargers from china I've tried are terrible for running hot and have no electrical safety approvals.

50Ah car battery with 5% self-discharge per month (flooded) is steady 3.5mA leakage current. Premium AGM are around 3%. It's the car accessories parasitic drain that is the problem, notorious Subaru's drawing 150mA caused a lot of grief.
If the battery is 50% drained, 500mA charging would take well over two days.
 

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,350
I did not mean to offend you, I just expected a lot of losses and since the transformer current needs to be half in order to prevent overheating, I though the LM317 will not be good, especially since most modules are only for voltage control and not for current control.
 

nsaspook

Joined Aug 27, 2009
7,296
I did not mean to offend you, I just expected a lot of losses and since the transformer current needs to be less in order to prevent overheating, I though the LM317 will not be good, especially since most modules are only for voltage control and not for current control.
There is no offense at all my friend. My comments were only directed at saving you time and money in the long run. I'm an expert at murdering batteries for fun and profit.

https://www.centurybatteries.ca/single-post/2016/05/06/Most-batteries-dont-die-they-are-murdered
Most batteries don't die, they are murdered
 
Last edited:

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,350
The first one is dead :D. Now I need to buy a second one. The seller at the shop tells me that it will get charged to 12.8V maximum and not to 14.4V. He also says that at 12.8V its 100% charged. I am not really certain in this, but I see some videos which show that the voltage from 14.4 will drop to 12.6-12.8 if you use the car lights for 2 minutes, which suggests the seller's information is correct. Has anyone actually tested this and can you explain it?

@Dodgydave sorry for not replying, I did not see your post.
 
Top