Maximum battery charger voltage allowed - The Unofficial BMW M5 Messageboard (m5board.com)

dhl93449 · 6th March 2008, 04:52

I have searched the archives of previous threads, but cannot seem to find an answer to this question:

How high can the battery charger voltage be when charging a battery installed in the car (with the ignition and everything off)? I ask becasue I just bought a Schumacher 8000 microprocessor charger, and this thing ramps up to well over 15-16 volts when trying to charge the battery. Most chargers I used in the past go to about 14.75-15 Volts max during the constant current bulk part of the cycle and then hold the voltage at about 14-15 volts during the absorption phase.

An indy mech told me the systems will shut down at 18 volts when running (in response to bad alternators/regulators), but he didn't know what the max input voltage would be with the car off. I am concerned because there are a lot of computers and electronics that are live on the battery even with the ign off.

I'm sure BMW has a spec for this, but I don't know how to find it.

BMW x 2 · 6th March 2008, 06:05

You need not worry. The voltage created by the charger will be pulled down to the voltage potential of the battery. A 16 volt charger will not create 16 volts to anything connected to the battery. These downstream components will only see whatever voltage the battery is at. The extra voltage is not a problem. They use the higher charger voltage to act as a quick charge and generally the volatge will fall off if the voltage at the battery is good. The current is a much bigger deal. Plus, a good charger will not trickle charge a good battery forever. It will bring the battery up to full potential and then the charger should cycle on and off. Don't sweat it. The 18 volts you were told is BS. The circuit will never get that high. This is unloaded voltage of the charger you are reading.

dhl93449 · 6th March 2008, 06:23

Quote:

Originally Posted by BMW x 2

You need not worry. The voltage created by the charger will be pulled down to the voltage potential of the battery. A 16 volt charger will not create 16 volts to anything connected to the battery. These downstream components will only see whatever voltage the battery is at. The extra voltage is not a problem. They use the higher charger voltage to act as a quick charge and generally the volatge will fall off if the voltage at the battery is good. The current is a much bigger deal. Plus, a good charger will not trickle charge a good battery forever. It will bring the battery up to full potential and then the charger should cycle on and off. Don't sweat it. The 18 volts you were told is BS. The circuit will never get that high. This is unloaded voltage of the charger you are reading.

I am reporting the voltage at the battery with the charger loaded and charging (not open circuit). Well, at least the voltage at the charging terminals in the engine bay, which will have a negligent voltage drop to the battery with a charging current of 30 amps (the assumed max current for this charger). I measured this with an accurate DVM. So this charger does raise the actual battery potential to 16+ Volts. I don't know exactly how much higher it would get, because I pulled the plug on the charger when the voltage got to 16V.

And I don't think 18 V is BS, because the guy I spoke with is very experienced, and you can check back posts of alternator/regulator failures that will fry batteries by overcharging them. If this happens, the normal low internal resistance of the battery is gone and the electrical system could see 18V.

BMW x 2 · 6th March 2008, 16:40

If you have 16VDC at the battery you have something seriously wrong! This is highly, highly unusual and virtually impossible with a small charger and a good battery. You can listen to whoever you want-in 50 years I have never seen this happen. If you have 16v at the battery then you were correct to pull the plug as I am surprised the battery did not explode. You were fortunate to catch it so quickly. Like I said, take whatever advice you want. I gave you a text book answer and that is that no charger of the trickle or 10amp or less potential should have the ability to raise the cell voltages that high to yield 16 volts at the battery. Try it on another battery and see for yourself and that is assuming your VOM is accurate as you say it is.

dhl93449 · 7th March 2008, 01:54

Quote:

Originally Posted by BMW x 2

If you have 16VDC at the battery you have something seriously wrong! This is highly, highly unusual and virtually impossible with a small charger and a good battery. You can listen to whoever you want-in 50 years I have never seen this happen. If you have 16v at the battery then you were correct to pull the plug as I am surprised the battery did not explode. You were fortunate to catch it so quickly. Like I said, take whatever advice you want. I gave you a text book answer and that is that no charger of the trickle or 10amp or less potential should have the ability to raise the cell voltages that high to yield 16 volts at the battery. Try it on another battery and see for yourself and that is assuming your VOM is accurate as you say it is.

This charger is no 10 amp or trickle charger. It is supposed to produce 30 amps and I believe it. I got it because my smaller 10 A charger cannot keep up with the drain when I am using accessories with the doors open, etc.

I called the Schumacher factory and described what was happening. They would not confirm that the 16+ volts is normal, but said it might be depending on the battery. They also would not tell me how high the voltage would get to because "it's controlled by the computer". They also claimed the charger is not defective, but I took it back for a refund anyway.

You are correct in that charging a battery of any type (AGM, gel, or flooded), with a voltage higher than 15 V is asking for trouble, which is why I pulled the plug at 16V. Apparently, to reduce charging time, Schumacher is force feeding high (30 A) current into the battery, and you need 16+ volts to do this, unless the battery is way down on charge. In my case, the battery read about 80% charged prior to charging, and the charger should not have been trying to push 30 A into it.

I was monitoring the voltage because this charger has a digital voltage readout on the front while it's charging. I couldn't believe the reading, so that is why I confirmed with a DVM at the terminals on the car, because I assumed they may have a large voltage drop due to cables. But in fact, the terminals in the engine bay were reading over 16 volts, confirming the reading on the front panel.

I attached the manual for the Schumacher charger if your interested. I am now going to be looking at a 20 A Xantrex charger that lets you manually control the voltages at the bulk charging stage (they are clamped at 14.2 to 14.5V) as well as the absorption and float stages. At least this company tells you exactly how much current and voltage are used when charging.

BMW x 2 · 7th March 2008, 02:58

First of all, I re-read my initial answer and realize how I came across. I'm sorry as I did not mean that 18V was not a maximum for the car. I felt it was BS that the battery would ever get there. A small, but significant difference in intent.
I am involved in a situation now with Generac- the makers of whole-house back-up power. I have not experienced this, but dozens of Generac owners have reported batteries exploding. The reason they explode is because they are "maintenance free" and as such the owner can not see and also can not add to the water level and because the internal charger never shuts off. It is constantly supplying a trickle charge to the battery even when the battery is at 100%. This causes the electrolyte to slowly "burn off". Once the level falls below the level of the plates, there is potential for the spark which is needed to ignite the vapor which has built up inside the battery case. The best chargers will not only fall off to a trickle, but will shut down completely as long as the battery is at its designed potential~13.6V. I believe I now understand what you are doing. You hook up the charger I take it so that you can run the car's accessories for long periods without starting the car? I thought you were just looking for a general charger which could bring the battery back up over time-not so! You are looking for something that can keep up with whatever you are demanding of the battery for longer periods.
Our cars certainly use more current than any other car I can think of, but that is generally with the car running. Because of the multiple pumps, lights, etc. in the BMW, it needs that 850-1000 amp battery. What are you running off the battery with the car off? Must be an awful big sound system to make the charger pump 30 amps constant into that battery.

dhl93449 · 7th March 2008, 05:01

Quote:

Originally Posted by BMW x 2

First of all, I re-read my initial answer and realize how I came across. I'm sorry as I did not mean that 18V was not a maximum for the car. I felt it was BS that the battery would ever get there. A small, but significant difference in intent.
I am involved in a situation now with Generac- the makers of whole-house back-up power. I have not experienced this, but dozens of Generac owners have reported batteries exploding. The reason they explode is because they are "maintenance free" and as such the owner can not see and also can not add to the water level and because the internal charger never shuts off. It is constantly supplying a trickle charge to the battery even when the battery is at 100%. This causes the electrolyte to slowly "burn off". Once the level falls below the level of the plates, there is potential for the spark which is needed to ignite the vapor which has built up inside the battery case. The best chargers will not only fall off to a trickle, but will shut down completely as long as the battery is at its designed potential~13.6V. I believe I now understand what you are doing. You hook up the charger I take it so that you can run the car's accessories for long periods without starting the car? I thought you were just looking for a general charger which could bring the battery back up over time-not so! You are looking for something that can keep up with whatever you are demanding of the battery for longer periods.
Our cars certainly use more current than any other car I can think of, but that is generally with the car running. Because of the multiple pumps, lights, etc. in the BMW, it needs that 850-1000 amp battery. What are you running off the battery with the car off? Must be an awful big sound system to make the charger pump 30 amps constant into that battery.

Interesting. I have a 3500 W Generac back up generator for my home. Got it when Enron energy traders were screwing California uitilty customers with rolling blackouts.

I have used a number of 10 A chargers on my cars over the years since some I do not drive often, and I want to keep the batteries topped up to as close as possible to 100%. They last the longest that way. When I got the M5 a month or so ago, I found these 10 A guys did not keep up with the current draw when I was working on the car, listening to the stock M Audio sound system, opening and closing doors, etc. One day after about 6 hrs with the key in pos 1 and the sound system on, the charger had to run full out for 6 more hours just to recover the lost charge. And that is with the charger on the car all day. So I wanted a higher current charger to cope with higher loads.

But any charger, if it's designed correctly, should not overcharge the battery, no matter what it's max current capability is. If it's a three stage charger, it should run it's max current until the absorption voltage is reached (14.2 to 14.9 V, depending on battery type, temp, etc), then it should go to constant voltage mode until the current drops to a predetermined value. The third stage is a float stage where the voltage may be held at 13 to 13.5 V (again depending on bat type, size, temp, yata yata) for the remainder of time. If during this period, a load is placed on the battery, I want the charger to maintain the float voltage level and just make-up the load current up to the load capacity of the charger. This will not overcharge the battery or cause excessive fluid loss in low manitenance or manitenace free batteries.

The Schumacher chargers apparently will not work this way. The Xantrex and other high quaility marine chargers work a lot better, and the 20 amp Xantrex charger has a "power supply" mode where it holds a constant 13.5 V after float and acts like a 20A DC power supply to back up the battery.

BMW x 2 · 7th March 2008, 17:25

Funny how we have had similar experiences! I have an M5 and a 540 and they both struggle to start after being worked on if I am listening to the radio, etc. I have never measured the current flow in position one, but when I was troubleshooting the Sec. air pump circuit I was in pos. 2 a lot and the draw is awesome. Usually requires an overnite charge!

30 amps though seems like a heckuva lot to push into that battery and not fall off quickly. Admittedly, I never charge while I am drawing the power-good idea, but I just by habit work on the car, drain the battery, and then hook up the charger. Mine has a 2-10-30-300 amp selection and even if the battery is dead to the degree that it won't crank fast enough to catch, if I put my charger in the 30 amp position it will swing up to 30 amps for just a matter of a minute and then I am at 10 amps or less and falling fast. I may linger at around 4-5 amps for several hours but by morning it is cycling on and off completely and I mean it shuts the charger off and then maybe 5 minutes later it comes on for a minute and shuts off again.

Generac, by the way, is one of the finest products I believe the average consumer can buy. Even though I had a bad stator once (under warrantee, but I still had to install) I love the security of 15,000 watts to run everything except my 5 ton a/c unit. It would start it, but I only had 12 circuits and some choices had to be made! Living in the country, septic pump, well pump, etc are "musts", AC is a luxury I can afford to be without for a while. But the charger circuit I removed and I use a battery tender brand charger which I really like. Pricey for the size, but truly a great logic circuit for what I am doing. Wouldn't help you a Bit though! I am just so happy with Generac products except for their charging system which could be mitigated by using a non-maintenance free battery. Nuff said. Sounds like you have found your nirvana and this problem is put to bed. You are to be commended for paying that kind of attention! Good for you and I'm glad you stayed with it until you got what you wanted. Schumacher is a well-known maker so I wonder if your particular unit was defective? The customer service though was poor at best. G/L my friend. I lived in Cal. for 5 years and i got the Whittier earth quake when I lived in Whittier, but left before we ever had a brown out.

Bill

dhl93449 · 7th March 2008, 19:44

Quote:

Originally Posted by BMW x 2

Funny how we have had similar experiences! I have an M5 and a 540 and they both struggle to start after being worked on if I am listening to the radio, etc. I have never measured the current flow in position one, but when I was troubleshooting the Sec. air pump circuit I was in pos. 2 a lot and the draw is awesome. Usually requires an overnite charge!

30 amps though seems like a heckuva lot to push into that battery and not fall off quickly. Admittedly, I never charge while I am drawing the power-good idea, but I just by habit work on the car, drain the battery, and then hook up the charger. Mine has a 2-10-30-300 amp selection and even if the battery is dead to the degree that it won't crank fast enough to catch, if I put my charger in the 30 amp position it will swing up to 30 amps for just a matter of a minute and then I am at 10 amps or less and falling fast. I may linger at around 4-5 amps for several hours but by morning it is cycling on and off completely and I mean it shuts the charger off and then maybe 5 minutes later it comes on for a minute and shuts off again.

Generac, by the way, is one of the finest products I believe the average consumer can buy. Even though I had a bad stator once (under warrantee, but I still had to install) I love the security of 15,000 watts to run everything except my 5 ton a/c unit. It would start it, but I only had 12 circuits and some choices had to be made! Living in the country, septic pump, well pump, etc are "musts", AC is a luxury I can afford to be without for a while. But the charger circuit I removed and I use a battery tender brand charger which I really like. Pricey for the size, but truly a great logic circuit for what I am doing. Wouldn't help you a Bit though! I am just so happy with Generac products except for their charging system which could be mitigated by using a non-maintenance free battery. Nuff said. Sounds like you have found your nirvana and this problem is put to bed. You are to be commended for paying that kind of attention! Good for you and I'm glad you stayed with it until you got what you wanted. Schumacher is a well-known maker so I wonder if your particular unit was defective? The customer service though was poor at best. G/L my friend. I lived in Cal. for 5 years and i got the Whittier earth quake when I lived in Whittier, but left before we ever had a brown out.

Bill

Thanks Bill. Based on your experience, it looks like a 30 A charge is on the high side for these batteries. Still am looking for the max input voltage spec, but I guess it's moot if anything over 16 V is going to be bad for the battery. Looks like the 20 A (or maybe a 15A) charger is what I need.

tstaples · 10th March 2008, 21:36

Just a follow up regarding voltage, based on 25 years automotive engineering experience. SAE recommended practices for all automotive 12V electronics are generally 6-16Vdc. In other words, with some exceptions at low voltage for non engine starting critical items, every 12V powered device on the car has to operate reliably from 6V (i.e. during cold crank on a nearly dead battery) up to 16Vdc (i.e. a running engine with a faulty Voltage regualtor.) I would not worry about anything on the engine being damaged by 16 Volts, except that your incandescant bulbs will age a bit faster.

That being said, 16V is a lot higher than anything I've ever see from any battery charger. Most top out in the 14 Volt range, when connected to the battery (i.e. loaded), and that only when the battery has reached a full charge. AT 16V acoss a conventional lead acid battery's terminals, the electolyte should be boiling away like crazy. It's just basic chemistry of the cell.

One thing to consider is the accuracy of the meter you used to measure that voltage. I've seen some lower end meters that were fairly accurate for AC measurement, but that were way off for DC measurement. I have a pocket sized EXTech that changes voltage readings on low voltage DC measurements depending on the charge of it's own battery. When the meter battery is fresh, 12Vdc will read close to 12V, but will rise to about a 15V reading as the battery is depeleted. Always question the measurement system!