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Hi appear may have a weak battery cell as the voltage may read ok with no load current, but when try to drive the motor will not have sufficient amps to hold acceleration speed. First would check the fluid levels of each battery and then check the wire connections are clean and secured.
reference manual (see pages 8-4. to 8-7.) ; http://www.yamahagolfcar.com/partsandservice/assets/pdf/om_0000000090_YWJ5A.pdf
Here are some helpful videos as well - http://www.yamahagolfcar.com/partsandservice/maintenanceVideoContent.aspx?type=consumer
If issue persist may find a local Yamaha dealer at this link - http://www.yamahagolfcar.com/dealer/default.aspx