I have high power (max 2000w RMS) battery powered amps (4x 3e audio tpa3255 amps) that I power from 3x nominal 14v lifepo4 batteries. But when I plug the battery in I get an arc on the SAE connector and it seems to trip the BMS. Unplugging and reconnecting reliably solves the issue but I worry one day it won’t and I’ll be stuck on the beach with my party friends and no sound! Any ideas how to solve this? Would a snubber or other inrush current management solution help? How do I specify the snubber components so that I don’t reduce the dynamic power supply capability of the battery (it’s about 40v and can supply 40a short term)
That sounds like a big inrush current.
How about a simpler solution: connecting the 4 amps one by one instead of all at once?
How about a simpler solution: connecting the 4 amps one by one instead of all at once?
For such a level of current/power, you should opt for a more deterministic method of connection: a good switch for example, that you flip once the connection is secured.
It could still be unsatisfactory though, because of the inrush current.
A big cap across the batteries might help, but the inrush current will be even higher and might weld or damage the connector or switch
It could still be unsatisfactory though, because of the inrush current.
A big cap across the batteries might help, but the inrush current will be even higher and might weld or damage the connector or switch
Switches that are designed to quench an arc are around, but won't be cheap and they will not stop the BMS from going into protection mode. Maybe, the big cap across the battery terminals is able to supply enough of the inrush current to prevent upsetting the BMS.
However, I still think that switching on the amps sequentially is the simplest and probably cheapest solution.
However, I still think that switching on the amps sequentially is the simplest and probably cheapest solution.
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You could always do a manual soft start with 2 switches. First one connects via a small resistor, 5 ohms maybe, and then throw the second switch to directly connect to the battery. Probably should turn off the switch that passes thru the resistor after turning on the 2nd switch.
Thanks for all the ideas. I tried connecting one amp at a time and that works great! I’m still curious to understand some more so I’ll do some more research on ‘inrush currents’. I guess this has to do with capacitors in the amps needing to charge up? Is there any risk of damage to the amps if it’s not tamed?
It has everything to do with those capacitors. When they're discharged, they're almost like a short circuit, when the batteries are connected a large current will flow and soon decrease as charge in the caps increases.
Instead of a snubber, you could implement a soft start circuit, a circuit that decreases the inrush current with some resistance and then bypasses itself when the charge is high enough.
I doubt the amps will suffer, but it won't be good for the contacts and I wonder about the BMS too.
Instead of a snubber, you could implement a soft start circuit, a circuit that decreases the inrush current with some resistance and then bypasses itself when the charge is high enough.
I doubt the amps will suffer, but it won't be good for the contacts and I wonder about the BMS too.
Yep, the connector contacts are being eroded rapidly! I’m wondering if a high current capacity relay that shorts across an in-line resistor after a time delay might solve the problem? I want to be sure that after the soft ‘start’ there’s no impediment to current supply - that’s one of the benefits of the battery power I don’t want to lose (apart from the main one… i can take the setup to the woods and the beach!) I have some time delay relays on hand but low power, I could have them trigger the main relay to bypass the resistor. Would something like a mosfet be even better? Or could that impede current peaks?
Another approach is to use NTC thermistors in series with the amp ac power, such as those in the data sheet below.
They are designed exactly for this purpose. Simpler to install than the relay circuit and very effective.
They are designed exactly for this purpose. Simpler to install than the relay circuit and very effective.
Thanks @techtool i had heard of the thermistor approach but was concerned it might limit peak current, looking again it looks like it would be fine because the temperature should remain high enough to keep the current flowing… it’s just whether I can find one with enough current capacity (40amps) and this is DC / battery powered not AC, but I don’t think that’s an issue. And being battery powered, efficiency is always a concern!