I believe, my situation is not unique. Many BMW users are facing the problem of a discharged battery. Accordingly – it was logically to conclude: there should be an appropriate charger!


I was looking for a compact SMPS-type charger. Preferably – with the possibility to manually set the parameters (taking in account, that the determination of correct SOC could be impossible due to low temperature or other consumers, which would load the battery, while the car is “going to sleep”). And, most important – increased voltage, because the car (and its battery) are in environmental temperatures, which can be heavily under 0 oC.


Checking all available (several tenths) options, most convenient for me looked: NOCO Genius G7200EU.

Unlike others, this charger had “winter” mode, several programs with increased voltage. For example, AGM+ promise voltage even till 15.5 V during charging, which should be acceptable in temperatures till -20 oC.

Today I performed some tests. Outside temperature has dropped below -10 oC. When if not now?

Unfortunately, there were no surprises. But, for all in a row.


SOC, indicated by the IBS system, was below 40 %, so – the battery very far from the charged state.

I locked the car, connected the charger. During first minutes (while the consumers of car’s electricity were active), the charger detected the state of the battery (variating the strength of the current 1.8/3.6/7.2 A and measuring a voltage of the battery) as if charged for 25 % and started to charge it. Everything looked very hopeful. Charging voltage reached 7.5 A – even a bit more than promised in Datasheet. More precisely I should say – it was current, advanced by a charger. The voltage on battery clips reached only 12.5 .. 13.0 V, which means – actually the car’s electronic was consuming the current from the battery (and charger), the battery was not charged.

Unfortunately, as soon (after 10 .. 15 minutes) the car’s electronic modules disconnected and the voltage on battery’s clips increased, the charged detected its charging status as 100 % charged and stopped the charging.

Unfortunately, the same scenario repeated in all charging programmes.

Note: increased voltage on battery clips (during charging) in cold weather is a normal occurrence. Electrochemical processes of battery in low temperatures are low-active, only outside parts of battery lead plates are “working”.


Unfortunately, my worst suspicions were correct. I would allow me to say – automatic chargers of this class are not able to “consume” such complicated tasks. And the fault is not directly in one or other charger.

For algorithms of automatic chargers, battery charging data “from the book” are used. Unfortunately, the automatically temperature-compensated mode is possible only in case if the charger’s clip is equipped with a temperature sensor – an such accessory is not common in chargers in range “around EUR 100”. In addition – even the most complicated electronics don’t help in detecting correct SOC, using the express method. Express method – method, which allows detecting SOC during a short time (during several seconds, several tenths of seconds or several minutes).

SOC, unfortunately, can not be measured directly, measured parameters (voltage, the internal resistance of the battery) are affected by temperature, charging “history” and other obstacles.

So, in the situation, when the IBS system detected, that the charge is below 40 % (close to critical minimum), automatic charger detected SOC as 100 %. Question: in which measurement (IBS or measured by the charger) to believe?


I believe in the IBS system. I have many arguments:

a) IBS system take into account the temperature of the battery (it’s measured all the time). So – IBS takes into account changes in voltage and changes in battery’s efficiency factor depending on temperature;

b) IBS system measures the voltage of the battery during a longer period of time (also in sleep mode, several hours after the finish of charging – then it’s possible to detect SOC more precisely);

c) only SOC system counts the energy, supplied to the battery and taken from a battery (including sleep mode), it means, the “history” of the battery is known for a long period of time.


In contrast, automatical charges for several hundreds of EUR don’t measure the temperature of the battery, don’t know its charging “history”, they don’t have time reserve of 2 hours, to (according to standard) measure the voltage of a battery and detect the SOC, according to these measurements. In addition, they don’t know, at which moment the load is “only” battery, in which moment – other consumers are connected (electronic modules).


My conclusion is harsh – automatical chargers are not suitable to maintain the car in winter conditions.


What to do in such a situation?

I chose to modify my charger a little bit. 90 % of time took the opening of its body, modifying itself: 10 minutes.

The essence of modification: to increase the threshold of the voltage for a cold mode.

Taking in account, that the “heart” of the charger is specialized MCU of Holtek HT46R67, which software I cannot modify, I had to implement an additional switch, with which to change the necessary charging mode. The switch turns on either the “normal” and “real winter” mode.

In a series of R45 (75 Ohm) I additionally connected 10k Ohm (which is shortened by a switch, when the “normal”” mode is chosen). This additional resistance increased allowed voltage of the battery for 15 %.

After “improvement” of charged I performed repeated tests. This time even after a car went to sleep, the charger detected the state of charge around 25 % and continued to charge it. When choosing modes, intended for AGM batteries, the charging voltage of 16.0 V could be reached, which is more than enough!


Here, after charging for 2 hours, IBS has recalculated SOC for 4 .. 5 % higher than starting the charging (for the result, we have to add several %, which were lost, while the car went to “sleep”.

Note: for batteries of last decade, the gassing is not a problem for batteries any more. This shortage (release of hydrogen and evaporation of electrolyte) is largely eliminated, for example, using a different number of lead plates for positive and negative electrodes.

Thanks to this, the increase of voltage is acceptable (very actual – in cold weather). In addition, the current (energy), flowing thru battery is not large (only several A) – by such small current the pronounced gas release would not be observed even for batteries of an older generation.


In the end – unfortunately, algorithms of all automatic chargers, observed by me (including Ctek), provides, that if the state of a battery is detected as “charged”, then for a long time (even till 10 days) the battery will not be charged even if:

a) it will be only partly charged (falsely detected SOC previously);

b) it will be discharged (for example, due to some reason the car will “wake up” and consume a lot of energy).

For charger to remeasure SOC (and, if necessary, to initiate the charge of battery), it has to be disconnected from the mains supply for a moment.

This nuance completely disables the option to normal use the automatic chargers in such (the battery is not disconnected from a car, of the temperature of the environment is far from optimal – around 20 oC) conditions.