Charging car battery and not disconnecting the "car". What would happen? - Page 1
Forum section: electronics / beginners

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 27, 2012, 04:23:15 am
content: I want to know what is the worst that can happen when you connect a battery charger and don't bother to untie one of the terminals of the car. A Kingwood, I only get to drive once every two months. Battery keeps going flat. I suspect it's the analogue clock in the 1970's GTS Holden dash.I have a 8A solar charger. I found an old tranny (no, a transformer- not transvestite) and I built a diode bridge/filter cap for the input instead of the intended solar panel. Seems to keep charged an 12v truck-sized battery I keep on a trolley (for emergency jump starts). The charger was bought off ebay or jaycar. Don't know which (funny how it's a bit hard to tell now).Anyway, the documentation of said charger says it will charge when the battery voltage falls below 10.5v and then cease at 14.2v. And will let the volts settle and drop to 10.5 before charging again.Bottom line: I have alligator clips on the charger and just want to plug it in without going to the effort of disconnecting battery terminal.When parked, the key is off. The radio is a switch-at-the-knob type. Coil type ignition, so no electronics to fry.What's the worst that can happen? Someone (in the pub) told me I could still fry the Alternator. Is this true?Or do you think I should just full the fuse to the damn clock? Or do I -really- need to disconnect the battery?

author: GEuser
author_rank: Frequent Contributor
timestamp: October 27, 2012, 04:35:00 am
content: There are diodes in the alternator , all cathodes pointing towards the battery , so if the alternator/rectifying diodes are ok in the first place there is no troubles at all ...imo

author: poptones
author_rank: Frequent Contributor
timestamp: October 27, 2012, 04:37:47 am
content: I'm 50 and I've worked on a lot of cars. The only time I've ever disconnected a battery to charge it is when I suspected there was a charging problem in the car and didn't have a battery tester handy.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 27, 2012, 05:30:46 am
content: Quote from: GEuser on October 27, 2012, 04:35:00 amThat's what I thought! Try explaining that to some drunk punter in the pub!

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 27, 2012, 05:32:18 am
content: Quote from: poptones on October 27, 2012, 04:37:47 amOk. Thanks. Want to know what, for example, would raise your suspicion.

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2012, 06:06:47 am
content: Battery goes flat if not driven for a day or three, or you want to get the charge current to the battery when running.Just connecting it to the battery via a 10A fuse will do the job, the fuse will prevent any oops from killing the electricals in the car. If the lighter is always powered then simply use a fused lighter lead to connect the charger, and run the cable out the window to make plugging it in easier. 2A through the lighter socket will keep the battery charged, but will not harm the wiring. Why not just use the panel as well, then you will have the battery charging during the day and it will cost nothing electricity wise.

author: MartinX
author_rank: Regular Contributor
timestamp: October 27, 2012, 07:19:17 am
content: Does the charger really let the voltage drop down to 10,5V? That is way too low, you should not let the voltage drop below 12,4V during log term storage to avoid damage to the battery, a float charger that holds the battery at 13,5V or thereabout is the best way to keep it charged.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 27, 2012, 07:25:58 am
content: Quote from: MartinX on October 27, 2012, 07:19:17 amSounds a bit low doesn't it? I will check my documentation. I could well have stuffed up the numbers.

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 27, 2012, 09:13:57 am
content: no, sounds about right...

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2012, 10:27:29 am
content: Normal float is 13.4 to 13.8 depending on temperature. Letting it go down to 10.5 and then charging to 14.2 will take the battery from 10% to 100% with each cycle, OK for a car battery where you inspect and fill water every service or every 3 months, but on a sealed battery this will kill it.

author: G7PSK
author_rank: Super Contributor
timestamp: October 27, 2012, 10:51:24 am
content: With old cars there is absolutely no problem, it was how it was done until they started putting all the electronic engine management systems in place, I suspect that charging the new cars would do no harm but the manufacturers put scare warnings in the hand book and on components in order to push you towards the dealer instead of solving the problem oneself.

author: krish2487
author_rank: Frequent Contributor
timestamp: October 27, 2012, 01:00:48 pm
content: One more aspect to keep in mind is the Ah rating of the car battery.As a rule of thumb, Lead Acid car batteries should not be charged at more than 10 - 15% of their Ah rating. All bets are off at anything above 30%.Solar chargers are not generally CC type as they are intended to "dump" most of the available solar energy into the battery.You have not mentioned the panel size you intend to use with the charger.Solar chargers can be used to charge a lead acid battery, but if and only if the short circuit current of the panel used with the charger does not exceed the 10 - 15% of the Ah rating.I am assuming (I live in india and i am not sure if the Ah rating is about the same everywhere ) that your battery is a 26Ah battery.For keeping the battery in good condition, you d need to charge it at 3 A max, which means you would need a 40-50 watt panel for use with the aforementioned chargerPS:Scratch that, i just saw the post about the transformer and all,it is OK to connect the charger with the battery connected to the car.Just make sure that the Diodes and the capacitors voltage ratings are high enough to allow for spikes during the turn-off and on of the alternator and the current ratings of the diode is good enough to handle about 8A.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2012, 01:24:01 pm
content: a good car electrical system will flat a fully charged (good condition) battery in a week or two. if it only took few days then something wrong. OP question... no problem charging while connected to car or at least i never saw bad things happened. i always do this when necessarry.

author: robrenz
author_rank: Super Contributor
timestamp: October 27, 2012, 01:55:59 pm
content: Warning!!! Many years ago I damaged the hood of a friends car when I had the charger connected and the hood almost closed because it was raining. The car had been sitting for several months and I was charging it to try to start the car. Well those explosive gas warnings are real. I heard the explosion and went out to see a severly buckled hood. I don't know if this still applies with the so called no maintenance batteries of today.

author: vl400
author_rank: Regular Contributor
timestamp: October 27, 2012, 02:11:46 pm
content: I have got a CTEK charger (lots of cheaper clones to choose from too) on all my seasonal or limited use toys. They have all been great as a full multi-stage charger and also maintenance charging. They are left permanently connected (battery still connected to the vehicle) and never had a problem with flat batteries since.

author: poptones
author_rank: Frequent Contributor
timestamp: October 27, 2012, 05:00:05 pm
content: That sounds like a sulphated battery. This is one of those reasons you should babysit a car when the battery is completely flat and you are charging at a high rate. A sulphated battery will usually be easy to spot as it stinks when it's charging.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 27, 2012, 05:12:59 pm
content: QuoteYes, charging standard lead acid batteries produces hydrogen & oxygen. You can hear it bubbling out on larger batteries when charging at high rates.Sealed batteries like AGM or gel cells should not let hydrogen outside the battery unless overcharged. But other "Maintenance Free" car batteries, like most car batteries these days, are just standard lead acid with non-removable (or difficult to remove) caps.

author: MartinX
author_rank: Regular Contributor
timestamp: October 28, 2012, 07:11:11 pm
content: Quote from: SeanB on October 27, 2012, 10:27:29 amAt 10,5V the charge state would be under 0% for a battery sitting idle, 10% at 10.5V must be for a battery during discharge not a battery sitting in an unused car. Anyway discharging a normal car battery to 10% is always a bad idea, their performance as a deep cycle battery is poor. Letting a car battery sit in an unused car and letting the battery voltage slowly dwindle down to 10,5V before recharging it will kill it for sure.Here is a link to a car battery charge state voltage table.http://www.mmbalmainauto.com.au/PDF/State_of_charge_12_volt_batteries.pdf