Charging car battery 12V - Page 1
Forum section: electronics / beginners

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 27, 2025, 10:56:14 am
content: Hello. I have a situation where I need to charge a car battery from medium/low to fullbut I don't have a voltmeterto check the voltage. I have heard that it should be 12.7V when it's full and 11.7V when it needs recharging. Is that true?The battery I have reads Pb, so perhaps it means lead acid? While I don't have a voltmeter, the charger has an amp-meter. Can I use the amp-meter to know when its full? For example, will the current drop below a level when it's full? Or is there no way to know when it's charged unless I get somebody to bring me a voltmeter? Best regards!

author: Psi
author_rank: Super Contributor
timestamp: January 27, 2025, 11:00:25 am
content: Safest way is to float charge it at 13.5-13.8V. It's safe because you can leave it at that voltage forever and you wont overcharge it.If you go higher than 13.8V it allows you to push more current in and change it faster but you risk overcharging it if the voltage gets too high, eg, you forget about it.So usually that is only done on smart chargers that know when to shut off.13.8V is the normal lead acid float voltage but some batteries may recommend 13.5V or something a little different.There is one important safety thing to remember when charging lead acid batteries. They can start 'gassing' which just means hydrogen gas is bubbling up inside the cells and possibly leaking out into the air. Hydrogen can be explosive if the air mix is right. Many people have been injured when disconnecting the battery from the charger if the terminals spark and ignite the hydrogen. In worst cases the battery can explode and throw acid everything. So ALWAYS disconnect the charger at the wall or at the powersupply. NOT at the battery terminals. That way any spark is not above the battery where hydrogen might be.Your device looks like a generic battery charger, so should be fine.

author: mikerj
author_rank: Super Contributor
timestamp: January 27, 2025, 11:24:25 am
content: Quote from: king.oslo on January 27, 2025, 10:56:14 amThose are resting voltages, under charge conditions the voltage should typically be limited somewhere in the range 2.30V to 2.35V per cell, though this does vary a little depending on the specific battery type e.g. AGM, gel and flooded and also varies with temperature. This is not a suitable float voltage though, as Psi has mentioned so there will need to be some way of terminating the higher charge rate.

author: CaptDon
author_rank: Super Contributor
timestamp: January 27, 2025, 02:22:05 pm
content: 12.7 is somewhat normal for a fully charged battery which has been resting for a few days. The charger you show in the picture will push the battery up to around 14.2 volts and the current will drop off to around 1 to 1.5 amps as the battery approaches full charge. At start of charge it will draw around 4 amps from that charger. If the battery is really dead it will start off with a low charge current, then ramp up and finally start to taper off. Although the charger shows pictures of motorcycles and so forth it is really only suited to the bigger 'group 24' style batteries. It will ruin smaller batteries like U1 garden tractor size or motorcycle batteries if connected for a long period of time. King Oslo..... I have seen the beauties sun bathing on the hillside beside the palace. I love the harbor area.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 27, 2025, 04:14:47 pm
content: Thanks for your replies.The battery is 100Ah and bought in 2025 so it's pretty new and also somewhat beefy. It sounds like it's not going to be a problem that am without a voltmeter, then?Is it correct that the idea with these devices is that users may cram charge into them until the ammeter falls to 1A and bob's your uncle?The meter has two DC amp scales. One reads "eff", and the other reads "arith"? As you can see in the photo, the eff scale has numbers that are 50% bigger than arith. Can you say what the idea with them are?Quote from: CaptDon on January 27, 2025, 02:22:05 pmSæll bróðir

author: Benta
author_rank: Super Contributor
timestamp: January 27, 2025, 06:06:45 pm
content: "I have a situation where I need to charge a car battery from medium/low to full"Not recommended. Car batteries are not built for that. You should not discharge a starter battery to lower than 80% of full charge, otherwise you'll get a very short lifetime.There are special "deep cycle" types available for your application (often used in recreational vehicles/campers).

author: CaptDon
author_rank: Super Contributor
timestamp: January 27, 2025, 07:53:41 pm
content: Takk fyrir!! Fyrirgefa, pessa lyklabord er ekki i islensk. Skal i botn brodir. I traveled to Norge during 1994 to visit a friend Jon Erik Martinssen. He was living in the town of Jessheim. I spent a few days walking around Oslo in the palace area, the city and the open marketplace along the long sidewalk. Sadly it was during the time of the 'Cod Wars' and I was wearing my 'Eimskip Reykjavik' jacket. Some fellows weren't exactly happy to see me. My stay however was very nice. I built a small shortwave transmitter for Jon and sent it to him from here in the states, however I am not sure if it worked very well as the transformers were designed for 60Hz and may have gone up in smoke at 50Hz. I think I sent along a 220VAC to 120VAC transformer style converter for EU / Skandinavia to US power conversion so he could power the unit.

author: J-R
author_rank: Super Contributor
timestamp: January 28, 2025, 01:20:14 am
content: "Lead acid" batteries can have slightly different characteristics, so always check with the manufacturer before making an assumption.For example, I have some Duracell Ultra branded 12V 9Ah (part DURHR12-9FR/SLAHR12-9FR, standard 7Ah form factor) and their resting voltage after weeks of sitting is significantly higher, such as 13.3V, yet the float charge is still suggested at 13.5-13.8V. They are labelled as AGM SLA.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: January 28, 2025, 03:54:46 am
content: Quote from: king.oslo on January 27, 2025, 10:56:14 amNot exactly. A normal wet cell lead-acid battery will be 12.6-12.7V when full and it should be recharged as soon as it drops below 12.6V. The reason is that it will slowly sulfate and lose capacity if it spends a long time below 12.6V. They like to be stored fully charged.QuotePb definitely means lead, but it may be a flooded (wet) cell (either allowing access for maintenance or "maintenance free", they still have liquid inside), or it may be AGM (absorbed glass mat) or SLA (sealed lead-acid). Both of these have slightly different charging requirements and this charger is designed for flooded/wet cell.QuoteYes. The charger manual says it supplies maximum 14.1V to the battery which is a good voltage for cycle-charging. The amp-meter will drop when it is fully charged.https://batteryuniversity.com/article/bu-403-charging-lead-acidYou can see here that the charge current will take several hours to finish dropping. Lead-acid batteries have an "absorption" or topping charge that takes several hours to complete. Don't leave this charger permanently connected to any battery but if the battery is low then it will take at least 6-12 hours to charge it fully, because of the absorption stage being so slow.

author: golden_labels
author_rank: Super Contributor
timestamp: January 28, 2025, 04:09:06 am
content: My family had a charger of that kind.(1)They’re good for letting the battery charge overnight. Or to pre-charge it, if you forgot to turn off lights while parking the car. Basically putting as much charge as possible in “some time”, but with no particular target.There is no smoothing filter after the rectifier, so the output waveform is not DC. The meter shows peak value (“eff.”) and the RMS value (“arit.”).Here’s a reverse-engineered schematic (modified by mecenas64 of elektroda.pl, from Einhell AFEN 10):Among attachments: the original Einhell AFEN 10 schematic for comparison.(1)Not the same model. I think our was the older AFEN series.