Ideal diode used with 2 batteries experiencing low voltage cutout

[ScooterBill]

I have a mobility scooter that I upgraded the controller on. It is a 1800 watt controller. I am using two 48V 10ah Lithium batteries in parallel with 50 amp ideal diodes ( https://www.amazon.ca/Anti-Reverse-...72/ref=sr_1_6?keywords=ideal+diode+50a&sr=8-6) for each battery. Under heavy load (going uphill) my controller will cut-out, display goes out and I must power it back on. My controller manufacturer suggested that the problem is that my batteries are not able to handle the output current needed and the voltage is dropping and the BMS is cutting out. If I run the batteries in parallel without the ideal diodes this does not happen. So what I am thinking is that the ideal diodes will only allow current to be drawn from the better battery (higher voltage) and not both batteries at the same time. I was thinking that if each battery is rated for a 30 amp output and I was drawing 45 amps it would just draw 1/2 from each battery. Is this wrong?

 

[scianiac]

I believe the answer is both, if both batteries are close enough to the same voltage than when drawing higher loads the higher voltage one will sag down to the lower voltage one which will switch on it's diode at which point they may both work together or may rapidly alternate depending on how exactly that little chip controlling the ideal diode works.

What do you have the ideal diodes? Yes they are required for some applications but it sounds like these batteries are pretty similar and probably stay on the scooter so if you leave them always connected together including when charging you shouldn't need the diodes.

 

[4πr^2]

Sounds like these ideal diodes are less than ideal!

I'm not specifically sure where that term comes in...I don't see it in the ad. Typically 'ideal diode' is a theoretical 'perfect' diode with no voltage drop. But I don't think that exists real world. So there is going to be some natural voltage drop across the diode. Typically around 0.7V for silicon. Not too bad, but maybe enough to put you over a tipping point.

I'd also be curious if there is an actual part number on the diode. Possibly you could get a datasheet with actual specs, then. Given an ad like that on amazon, it is possible the diode is rated for 50A max and something much lower nominal. Even if the diode is 50A nominal, then you've got the circuit board, mounts and wire lugs all adding resistance to the circuit plus some expected diode drop.

As mentioned above, depends on how close your batteries are. But you say you have already run the bike without the diodes...and doesn't seem like anything blew there?

 

[scianiac]

"Ideal" diodes are not actually diodes, they are a mosfet that is controlled by a chip so that when the mosfet is driven on the voltage drop is negligible, well technically it's based on the RDSon of the mosfet. The issue I wonder about is what happens when battery A is being drawn on so battery B is but cut off by by it's doide then the battery A voltage drops to match B and B's diode turns on but then with less power being drawn from A does it's voltage rises and B diode turns off or does it oscillate back and forth with diodes turning off and on rapidly based on whatever the reaction time and sensitivity of the little chip controlling the diode fets is?

 

[ScooterBill]

The batteries are the same, but one has been used a bit longer and tests at about 350 watt/hours where the other test at 450 watt/hours, so I was originally concerned about the poorer one drawing down the better one. They do stay on the scooter all of the time, with the ideal diodes I would charge them separately, but have now been using them without the diodes and charging them together. Am I better just leave running them together with the possibility of lower total watt/hours but less peak current draw on each battery. I had thought that the diodes would just let the stronger battery get used 1st until it dropped down to the same as the weaker one and then they would both be used, but if the diodes just keep cutting the batteries in and out without drawing from both together then that isn't "ideal" anyway.

 

[glennb]

Sorry if this is unhelpful - I barely skimmed the thread - but the 350 and 450 numbers jumped out at me. They’re tiny batteries. IMO too small to bother with duplicated BMS or blenders. I’d want to merge them into a single larger pack, set them aside for a little kick scooter to go to the ice cream shop, or trade them on and buy a larger one.