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Old 14-08-2019, 07:43 PM   #5
GTLEGEND
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Join Date: Mar 2013
Posts: 7,471
Valued Contributor: For members whose non technical contributions are worthy of recognition. - Issue reason: Every advice post I visit where OP are seeking advice for their new vehicle, he is there and adds valuable support and advice for rectifying the problem. Really is a Legend. Technical Contributor: For members who share their technical expertise. - Issue reason: The quality of information he provides is in depth and highly valued by those posting questions. 
Default Re: focus battery replacement

A lesson on Battery Monitoring Sensor (BMS).

BMS is utilised by vehicles that are equipped with Smart Regenerative Charging (SRC) or Start/Stop system because these systems require data of the battery State of Charge (SoC) and State of Health (SoH). BMS is the sensor utilised to provide this data.

BMS reset should only be done when a battery is replaced and at no other time. The reset needs to be done via a diagnostic tool.

Any external electrical earths must not be connected to the battery earth post/terminal directly but at another earth point like the battery earth lead point on the chassis etc. Also the BMS pole clamp should not be removed unless the battery is being replaced, earth isolation should be by disconnecting the earth lead chassis eyelet. This is to prevent BMS data inaccuracy.

Connector to BMS is made up of two wires, one for LIN data bus, the other a 12V battery feed to provide power to the BMS. If the BMS needs to be removed the connector is to be disconnected first and connected last after the battery or connector refitted to the battery.

The BMS monitors voltage, current and temp of the battery to calculate the SoC and SoH. To function with accuracy, the BMS recalibrates itself at regular intervals. Recalibration occurs during a rest period when the battery quiescent current is less than 100mA for at least 3 hours. The longer the rest period, the better the accuracy. Recalibration must take place within every 7 days or the SoC cannot be guaranteed. This will result in the Start/Stop and SCR system being deactivated. These systems will also be disabled if the sensor detects the battery is reaching its end of life, low state of charge or health.

The BMS is unable to detect what type or size battery is built into the vehicle, so battery replacement should be one of the same size and type as that fitted from factory.

The BMS is also unable to detect the difference between replacement of an old battery with a new one, so a BMS reset is required to clear any accumulated battery aging data as it does not do it automatically. Failure to reset the BMS after replacing a battery can result in BMS estimating incorrect parameters and disabling the SRC and the Start/Stop system.

A BMS reset how ever does not reset the SoC level, only the accumulated aging data. To reset the SoC is done by disconnecting the power to the BMS and reconnecting it (known as Power-on-reset).

When charging a battery on car, you must not connect to the battery negative post but to the designated earth point. By charging directly to the negative post creates a blind charge because the BMS is unable to see the charge entering the battery. If the BMS does not see current flowing in or out of battery, it cannot correctly calculate an accurate SoC.

If the battery is charged out of the car, when the battery is refitted the BMS will go through a power on initialisation process for a few seconds to establish a quick and rough SoC estimate. it will take a minimum of 3 hours of rest as advised above for the BMS to re-monitor the battery charge level to a higher level of accuracy.

Also when reconnecting a BMS make sure there are no high discharge loads on the battery such as having the headlights on. High loads will cause a voltage dip at the battery which can result in the BMS reporting a very low SoC. Start/Stop and SRC may be deactivated until the rest period described above is seen.

Never reconnect the BMS when a battery is charging as it will also cause inaccurate BMS data.
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