My Fluke 199C had a noisy fan and a worn battery, so I fixed the fan and converted the battery from nickel metal hydride to lithium ion.
The original BP190 uses six 1.2 V nickel metal hydride cells, giving it a nominal voltage of 7.2 V.
For the replacement I used four lithium ion cells in a 2S2P configuration. The two series groups give a nominal voltage of 7.4 V and reach 8.4 V when fully charged, which is very close to the voltage range of the original battery.
The pack includes a BMS and an NTC temperature sensor. The BMS provides charge and discharge protection, while the NTC allows the ScopeMeter to monitor the battery temperature.
I modified the charging circuit using the 120 kOhm resistor shown in the photos. The final charging voltage was approximately 8.38 V, just below the 8.4 V maximum of the lithium ion pack.
I tested the complete charging cycle and then operated the ScopeMeter while monitoring the battery voltage. The ScopeMeter shut itself down before the pack became completely discharged, with the BMS providing additional protection.
The external power supply is slightly unusual. It produces approximately 17.8 V and has positive on the outside of the connector.
To fix the noisy fan, I peeled back its sticker and pushed some grease into the bearing. This stopped the noise without replacing the fan.
Photos of the charging test, fan repair and 2S2P battery pack are below.
Photos and videos
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