Backup battery system alternative diode device.

[DaveH]

Now that my F5J season has ended, I have finally got round to fitting out my remaining F5J gliders with a backup battery. 

 The usual diode for this job is the SM74611 Smart bypass diode.    This is the best device to use for a low voltage drop across the diode, particularly when using a single LiPo cell as the backup.    I have found it difficult to buy any, long wait times are often quoted with fairly large minimum order numbers.     Maybe others are interested in this solution.     

I have recently tried this device,  cheap, from Aliexpress.    

Despite the rather flowery description, it's a small cct board that operates as a single diode. It has very low forward voltage loss (~20mV).  It is under 3g , 22x27x4mm. 

My bench-top rig works very well (presently using a 2s battery backup). 

 

https://www.aliexpress.com/item/1005005394650427.html?spm=a2g0o.productlist.main.9.61f135e0hqnoes&algo_pvid=4953159e-5029-407e-a968-70a5215265ff&aem_p4p_detail=20231114040451230408326432200004570642&algo_exp_id=4953159e-5029-407e-a968-70a5215265ff-4&pdp_npi=4%40dis!GBP!1.53!1.44!!!13.29!!%40211b613916999634916807706e5438!12000032885167459!sea!GB!2748536064!&curPageLogUid=jzGWsg0dzSrj&search_p4p_id=20231114040451230408326432200004570642_5

[DaveH]

"Anti reverse irrigation"    LOL...    it took some time to find this item by using a sensible functional description!    

 

[Jon_S]

The Texas instrument SM74611 is an ideal diode controller. 
it acts monitors the two separate rails and along with a pair of MOSFETS is works to actively enhance the gate of the mosfet that switches in the higher voltage (in this case the battery back up battery). 
the series volt drop is kept low by using mosfets as switches and fully enhancing the gates to get the best conduction with minimum ‘on’ resistance of the mosfet (keeping the on resistance low prevents self heating if the mosfet itself). 
ideal diodes controllers are cracking little devices. 

[slopenut101]

Hi Guys

Very interesting

I have used two diodes and two  batteries on large scale gliders could I use 2 of  these in place of the  2 diodes?

The SM74611 as three pins  is one pin a redundant pin?

Cheers 

Ian 

[Jon_S]

Two diode will mean that both packs are depleted together. And the forward voltage of the diodes reduces the margin of your pack. 
remember the diode forward voltage increases as the temperature gets cold and as the current increases. 
the ideal diode driver using mosfets. 
sounds like there is an opening for a design for large scale gliders for back up battery controllers. 
it’s something I could design and get some pcbs made of people are interested. 
what are the requirements ?

[DaveH]

On 19/11/2023 at 13:39, slopenut101 said:

 

I have used two diodes and two  batteries on large scale gliders could I use 2 of  these in place of the  2 diodes?

The SM74611 as three pins  is one pin a redundant pin?

Cheers 

Ian 

I understand that the SM74611operates as a two pin device and can be considered as a drop in replacement for a diode (similar specs) and is actually a small circuit in one package .    The "Anti reverse irrigation" (love the name!) device described above is also a replacement for a diode, but has an additional 0v connection to power the circuit (the SM74611 powers itself differently).   

You will be able to use these devices if the spec suits, but will have to add a 0v connection to the boards. The 0v connection will not carry any power other than the minute amount needed to operate the little cct boards.  

 

[slopenut101]

Hi

Started using 2 x 5 cell nical metal hydride batteries and 2x Schottky diodes with standard servos 20 years ago

Moved on to A123 life cells plus two diodes useing standard servos

The latest  plane uses 2 x 2S  5000mha lion cells and 2 x 10 amp diodes and HV servos 

I know one can spend £100 plus on a propriety product but two diodes are cheap, bomb proof, the volt drop in the past was an advantage  Normally I use such large batteries that efficiency does not matter. It is better than carrying lead. 

Was hoping 2 of these could be used to replace the  diodes so full voltage would go to Hv servos  on the Lion powered glider but they work fine as they are so may not bother

Thanks for info

Ian

 

[DaveH]

You are using diodes to make a common connection directly to the receiver, (rather than a battery of twice the capacity).   

The Schottky diode is quite resistive, it may have a low Vf  (forward voltage drop) on low load and increase proportionally more than a standard diode/rectifier as the load fluctuates.  In practice, you will have a supply that carries more  electrical noise than a direct connection.    Using the diode cct, your supply will not drop by much (~0.02v) and will be much smoother.    I'd try a couple out, just wire each battery (+ & -) via its own little board.  Fit heatshrink covering to avoid any unwanted contact and excitement.      I don't know if you would see obvious signs of improvement.