GEN 4 Disconnecting the Battery Sensor

Gadget wrote:

I've never charged the battery on my Gen4. Is this a common issue or something specific to your car?

 

If you are driving every day and doing 100 miles a week or more you won't have a problem.

The BMS (battery management system) only replaces the charge taken out starting the engine, then reduces the alternator output to provide a tiny trickle charge, so if you drive 5 hours or so a week that is enough to charge the battery.

(There is a sensor in between the negative terminal and the earth lead that uses a very low value resistor to measure current and checks the battery voltage to report battery condition to the BMS, we are disconnecting the cable that reports the sensor readings to the BMS)

Those of us who walk everywhere and only get the car out to go long distances or carry bulky items find that the natural self discharge and parasitic draw from all those computers, the receiver for the remote locking, the alarm etc. will gradually run the battery down, and as Roger says it needs charging from an external source about monthly.

(I don't know exactly how the Suzuki one is set up, if you use a winch or loads of lights when the engine is running it ought to be able to figure out what came out of the battery and then replace it, it might keep tabs on large loads with the engine off too, but it can't deal with self discharge and small parasitic draws)


It is a common issue with "modern" cars that are only driven short distances, particularly EVs and hybrids which have a small capacity 12V battery that is vital to activate the contactors for the high voltage traction battery. They actually need a "jump start" to close the contactor and connect the (fully charged) main battery if they have been standing for a few weeks.

 

Thanks. Over 7 years I've averaged about 130 miles/week, so not an issue I've had.

facade wrote:

If you are driving every day and doing 100 miles a week or more you won't have a problem.
 

As previously mentioned, my wife's car is driven only on weekends, and averages around 30kms (less than 20 miles) per week - and has started every time she turns the key.  December 20 will make two years on the current battery which has never been charged other than by the Jimny's alternator.

facade wrote:

The BMS (battery management system) only replaces the charge taken out starting the engine, then reduces the alternator output to provide a tiny trickle charge, so if you drive 5 hours or so a week that is enough to charge the battery.
 

The Jimny does not have a battery management system, what it does have is an ECU controlled alternator - the current sensor on the battery negative, which by the way, is a hall effect sensor, not a resistor shunt, measures the current flow in/out of the battery, and the ECU uses the data to adjust the alternator output to match the vehicle's electrical load - in a nutshell, current must not flow from the battery, if the current is flowing from the battery the ECU will increase the alternator output until the current flow is reversed.

The battery has a three fold purpose, the first is to start the car, the second is to provide a source of electrical power when the alternator is unable, for whatever reason, to do so, and the third is to stabilize the system voltage - please note that third purpose - the alternator voltage regulator does not actually regulate the system voltage, it adjusts the alternator output current, that current into the battery is what determines the system voltage.

This last is also reason why the battery must NEVER be disconnected with the engine running, the system voltage will spike and can damage the vehicle electronics, it was permissible as a way of testing generator output back whan cars had DC generators and carburetors, but if done on a newer car, the risk of damage to the electronics systems is high.

Typical recharge time to replenish the energy used to start the engine is between fifteen & thirty seconds (you can measure this yourself if you have access to the proper instrumentation) after which the alternator only needs to output enough current to supply the vehicle's electrical load at that time, there's no need to have it producing a higher level of current, this actually increases the load on the engine, which bumps the fuel consumption up, and that is the reason that vehicles are now equipped with "smart alternators", the technology is not new, it's been around for almost thirty years.

All of which is fine except for the depletion of the battery by small losses when the car is off. With smart charging the average ecu doesn't observe the small losses when it is dormant and assumes that it is being driven regularly and frequently and as you say puts back what was needed for the start and the immediate running. Add to this that even now exactly how a battery can be assessed for state of charge is not fully understood and it becomes apparent that smart charging is not always the best answer. For reference I have been chasing early battery death on my land cruiser for nearly 6 years and have had to read up on far more dry academic papers and consult way more experts on the subject than any civilian should be expected to and all I have been able to determine with any certainty is to either leave it on charge between uses or disconnect the battery both of which are a little inconvenient and quite caveman solutions.

fordem wrote:

<snip>

Typical recharge time to replenish the energy used to start the engine is between fifteen & thirty seconds (you can measure this yourself if you have access to the proper instrumentation) after which the alternator only needs to output enough current to supply the vehicle's electrical load at that time, there's no need to have it producing a higher level of current, this actually increases the load on the engine, which bumps the fuel consumption up, and that is the reason that vehicles are now equipped with "smart alternators", the technology is not new, it's been around for almost thirty years.
 

This is the part I fail to understand.

If the alternator just constant voltaged the battery (with temperature compensation if it must...), like in The Olden Days then the current would fall away to near zero anyway as the battery charged, and the system voltage would be controlled by the alternator at around 14V.

If the system is ensuring zero current out or into the battery, then once the initial charge to replenish starting is over, the system voltage will fall gradually to that of the battery when standing unused- in the mid twelves. It ought to be able to then determine that the battery is in a low state of charge and do something about it, but we are talking about hours to get to a stable state, which is why it can never work with a small usage cycle. 

I guess it actually aims for a low charge current, say 100mA into the battery, which will eventually bring the battery voltage up to 14V, given infinite time- then the battery is controlling the system voltage, not the alternator.


Really I'd love to see the full technical description of the system and it's component parts.