DC-DC across battery (for lights, etc) quiescent current

[8o887]

Hi all,

Can't find discussion of this relatively simple, though potentially important, question:

I've read advice that a cheap standard DC-DC (eg 50V -> 12V) converter can be connected directly across the battery when the controller can't supply a couple of amps for peripherals. This seems to place a constant quiescent drain on the battery of up to about 5-10mA.
One thing I've never known is whether the BMS will typically shut off the output when low voltage is detected. A similar point arises with the quiescent drain due to the controller itself, considering that a bike might be stored for months (years?) without charging. Many designs do not include switches on the main circuit.

Comments?
Thanks in advance.

 

[E-HP]

One thing I've never known is whether the BMS will typically shut off the output when low voltage is detected. A similar point arises with the quiescent drain due to the controller itself, considering that a bike might be stored for months (years?) without charging.

I wouldn't leave anything connected to the battery if you're planning on storing the bike, especially the controller. I also wouldn't rely on the BMS in those situations.
I accidentally left my bike on a couple of weeks ago, and the voltage dropped from 81V to 79V after about 4 days. That was with the controller and CA display on.

 

[InfamousFruitHunter]

Using a buck/step down converter across the battery is fine, but I highly recommend adding a fuse (in case of the converter going short-circuit) and a switch (to shutoff the "lights, etc", as well as the buck converter itself) for when they're not in use.

I recommend a basic wiring diagram as follows:

Battery + --> Fuse --> Switch --> Buck converter Input +.
Battery - --> Buck Converter Input

This basic wiring diagram adds safety to the circuit and prevents the risk of over-discharging the battery.

 

[8o887]

The switch in the + lead? My instinct would be to put it in the return lead, to remove the 50V from the switch cables. Was thinking about a handlebar-mounted rocker...can you elaborate the logic? Thanks.

 

[MadRhino]

My bikes are sleeping with battery unplugged, matter of safety. The best step down DC-DC converter that I have used is Vicor.

 

[john61ct]

I wish I knew people that could turn the huge variety of Vicor units

into consumer -safe finished devices

 

[john61ct]

yes easily isolating the battery completely from all circuitry is IMO a must

including the BMS if any.

Too much to go wrong

not just reducing self-discharge rate.

 

[amberwolf]

The switch in the + lead? My instinct would be to put it in the return lead, to remove the 50V from the switch cables.

If the bike frame (or whatever the cables run along, and the switch is mounted to) is not grounded to that same return (B-) lead, then there is no potential between the cables and the frame, whether you use the B+ or the B-.

If the bike frame *is* grounded to the B-, *then* you have the battery voltage potential from the leads to the frame, if the switch controls the B+.

The same voltage potential will be across the switch contacts and wires, regardless of which side you put it in, whenever the switch is off. Meaning, when the switch is off, there's no longer a connection between the two wires, so the wire that goes to the DC-DC can have the full voltage potential to it, compared to the wire that goes to the B-. Whether a hazard is present or not to say, touching a wire accidentally, depends on the DC-DC input stage design, but basically if you were to directly short that wire that went to the DC-DC from the switch to a grounded frame (or whatever) that is also connected to the B-, it is going to allow current to flow as if you had turned the switch on, exactly as if the wiring were to the B+ side instead.

Which one, if either, is "better" depends on the specific wiring of the rest of the bike, etc.

 

[E-HP]

The same voltage potential will be across the switch contacts and wires, regardless of which side you put it in, whenever the switch is off. Meaning, when the switch is off, there's no longer a connection between the two wires, so the wire that goes to the DC-DC can have the full voltage potential to it, compared to the wire that goes to the B-.

Another way to look at it conceptually, is that the two components basically make up a voltage divider. The switch resistance is fixed, in milliohms, and the converter is a variable load (5k ohm on the high end if the converter pulls 10mA while idle), but regardless of their placement, the potential across each remains the same.

 

[LewTwo]

A similar point arises with the quiescent drain due to the controller itself, considering that a bike might be stored for months (years?) without charging. Many designs do not include switches on the main circuit.

That is a Excellent point.

My six year old Golden Motor battery pack includes a main switch. I am certain it is NOT ahead of the BMS but that has not proven to have been a problem. It is however a little less peppy than it was when new. For that system I just added a DC-DC converter and a couple of LED lights to the back end. Anytime the main switch is powered on so are the tail lights. I like that approach.

However the other two packs I have been working with have no switch except for the XT60 power plugs. My shark pack has been down for months while I procrastinated. Turns out to be a BMS problem and I have new one coming in this week. I am seriously considering mounting it in the heat sink of the shark mount (external to the main pack). I burned up the shark's stab connectors while disassembling the main pack ... sparks, fireworks, magic smoke, heart palpitations, etc. Thus I will need to insert a XT60 Plug anyway.

That brings me to my point. I am thinking that I might be able to mount a 'DC Circuit Breaker' where the BMS was previously installed. That would eliminate any chance of 'phantom power drain' from the BMS. This a 'common lead' DALY BMS so the motor/bike would need to disconnected and the switch "ON" to charge it. DC-DC converter and taillight will be similar to set up on my other bike (i.e. downstream of the main switch and always on).

Circuit Breaker: https://www.amazon.com/dp/B01JE7SRP4

 

[john61ct]

CB can be used as a switch

but cycling lifespan is much lower than a decent quality switch designed as a switch.

You do not need a massive expensive HD switch, so long as high currents are not flowing when you throw it.

And rigging a simple "arming plug" will do just as well.

 

[LewTwo]

And rigging a simple "arming plug" will do just as well.

H'mm ... had not thought of that.
Easy to stick in ones pocket when leaving the bike parked as well.
Good idea ... thank thee.