I believe the use of the diodes is to prevent any feedback into a weaker battery when 2 are in use in parallel and thereby causing it to overheat and cause a fire. It is always advisable to charge only one battery at a time and never in parallel

I have used two 7.2v battery packs connected in parallel for some years without any problems. I made a Y lead with Tamiya plugs and sockets. No diodes or other 'extras'. When charging I separate the batteries from the Y lead and charge them separately. This is just my choice, although they could be charged through the Y lead as Martin has done.
Take care to connect the correct leads and you should have no problems. Good luck, and happy sailing !😉

Thanks Doug on the info for the Schottky diodes. Did not know about them or the parallel power boards. Good to know about.

Hi can you post me photos b
how it’s done thanks

Hi on my crawler and tug. I have two 7.2v batts wired in parralel which gives me still 7.2v but the extra stamina. Don't do it in series it will double the voltage to 14.4 I have joined the two batts together and and charge them both at same time. Charger handles it. Hope that helps. Martin.

Absolutely right great advice

Hi thanks for the answer reason I ask the model has a Y harness and two Nimih battery’s but not one of those boards

Hi Stephen,
Basically; connecting in series doubles the voltage at the same capacity.
Connecting in parallel doubles the capacity at the same voltage.
Thus to give a longer duration time you would need to connect your batteries in parallel, i.e. to and - to -. Two 5000mAh 7.4V batts would then give a total 10000mAh of 'nominal' capacity at 7.4V.
BUT: to do this safely both batteries MUST be the same type (make and preferably production batch) and the same state of charge.
Precautions must be taken to prevent a stronger battery from trying to charge the weaker one and maybe causing high back currents to flow which can cause a wiring fire and loss of the model.
This can be especially dangerous when using LiPo batteries.
The two batteries need to be connected via fast Schottky diodes to prevent this.
If you are not au fait with electronics and soldering I can recommend this simple solution;
a 'Parallel Power Board' from Action Electronics, sold by Component Shop, see link below.
Open the link and click on [PDF DATA SHEET] for a complete explanation.
Cheers, Doug 😎
https://www.componentshop.co.uk/p103-parallel-power-board.html.html