Old Battery charger - Page 1
Forum section: electronics / repair

author: filtatosv6
author_rank: Newbie
timestamp: October 15, 2015, 07:39:10 pm
content: Hi I recently got an very old battery charger with variable output that had a problem with the bridge rectifier and the volt meter.There is no capacitor inside so the output is 100Hz rectified sin wave.I was trying to calibrate its volt meter when I realized that I don't know where to calibrate it.I wonder what the battery sees. Is it the RMS?, the Vmax? Or what I read in DMM in DC mode???e.g. in the first setting the output is17Vmax12V RMS10.5V DCIf for the sake of the argument I wont to charge a car battery what output do I need???ps i hope i explained it clear enough. English is not my native language.

author: tautech
author_rank: Super Contributor
timestamp: October 15, 2015, 08:00:05 pm
content: Welcome to the forum.Interesting it has a Voltmeter, normally an Ammeter is used to monitor charging current.For general car battery charging up to 14.5V is not considered excessive, although I tend to stay just below that @~14.2V. Over 15V is called a gassing charge and is sonetimes used for Automotive battery maintenence. This requires replenishment of lost water as a result of the gassing/bubbling.

author: oldway
author_rank: Super Contributor
timestamp: October 16, 2015, 10:55:43 am
content: Voltmeter is provided to measure battery voltage and must be calibrated with DC. (for exemple from an external power supply).When battery is connected, output voltage of the battery charger is filtered by the battery and ripple is low.You said that this battery charger has "variable output", I think you means that there are several taps on the transformer with a comutator to select one of them.This is intended to ajust charging current.As there is no voltage regulation, battery voltage of a fully charged battery will be close to the peak output voltage of the battery charger.This is often too high for modern batteries. (as Tautech said, may never be higher than 15V !)

author: filtatosv6
author_rank: Newbie
timestamp: October 20, 2015, 06:46:36 pm
content: Quote from: tautech on October 15, 2015, 08:00:05 pmSorry for my absence.Thank you for the welcome. I didn't say anything because I don't feel new to this forum. I am reading it as a quest for some time now.Quote from: oldway on October 16, 2015, 10:55:43 amYou got it right although the taps from the transformer give different volt outputs, up to 30V (Vp much higher) and the meters ware for AC. I wouldn't trust itAny way thank you for your answers.

author: oldway
author_rank: Super Contributor
timestamp: October 21, 2015, 07:01:28 am
content: QuoteThat is no load voltage and it does not mean nothing because secundary voltage is limited by the battery.Transformers for battery chargers have a high short circuit voltage (above 20%) to limit charging current.NB: short circuit voltage is how much % of nominal voltage you have to apply to the primary to obtain the nominal current with the short circuited secundary.In normal working, you switch the taps to ajust the charging current.AC voltmeter (moving iron) may also measure DC voltage.What you should look for is if it measuring voltage before or after the rectifier.

author: filtatosv6
author_rank: Newbie
timestamp: October 21, 2015, 07:55:30 am
content: Definitely after the rectifier. It is parallel with the output.

author: oldway
author_rank: Super Contributor
timestamp: October 21, 2015, 08:29:17 am
content: As I said, this voltmeter is intended to measure the battery voltage and must be calibrated with DC voltage from an external power supply. (battery charger NOT powered on !)When battery voltage reach 14V, you must stop charging the battery.

author: mikerj
author_rank: Super Contributor
timestamp: October 21, 2015, 10:26:54 am
content: Quote from: oldway on October 21, 2015, 07:01:28 amI've never heard leakage inductance described in those terms, is this an industry standard measurement?

author: oldway
author_rank: Super Contributor
timestamp: October 21, 2015, 10:54:42 am
content: Quotehttps://www.physicsforums.com/threads/impedance-voltage-of-a-transformer.571998/Ucc = short circuit voltage.See specifications of transformers:For example,http://www.erea.be/frontend/files/certificates/72_catalog-en-ed-2013.pdfThis is a transformer from a 12V 6A battery charger.Primary and secundary are wound side by side to increase magnetic leakage and short circuit voltage. (more or less 20%)