KU93 with Bafang CST 250w (350w) inadequate for hills

[Russell]

I'm using one of these https://drive.google.com/open?id=0B7ym9WvyF_P7ZTVpRG9uNE5SMWs

It's wierd but the Vmin was only 0.5v below Vrest at the end of the ride. Maybe I viewed it wrong, will try again tomorrow.

Really appreciate your other advice just now.

The 48v is a bit of a revelation on hills. Although I can see what people mean about it being inefficient below 50% of top speed.

I can't access the picture.

That difference between Vrest and Vmin is of course not correct. My logs indicate an internal resistance of 0.184 ohms for a voltage sag of 0.184V per Amp or just over 4.0V at 22A peak load. This relationship holds until almost 90% capacity has been used.

There are quite a few inline wattmeters available nowadays. I still consider the Watt's Up to be the best, though it is a bit pricey and not the most robust. I bought two Turnigy wattmeters for less but only one matched the WU meter's Wh and Ah values within a small margin, the other was way off. I tossed the second one in the trash. I tried an even cheaper blue wattmeter and it calculates the Ah/Wh about 8% low compared to my other two plus it gives instantaneous peak current values 25% above my other two wattmeters. And an inaccurate wattmeter isn't much good to anyone.

-R

 

[jonathan75]

I can't access the picture.

That difference between Vrest and Vmin is of course not correct. My logs indicate an internal resistance of 0.184 ohms for a voltage sag of 0.184V per Amp or just over 4.0V at 22A peak load. This relationship holds until almost 90% capacity has been used.

There are quite a few inline wattmeters available nowadays. I still consider the Watt's Up to be the best, though it is a bit pricey and not the most robust. I bought two Turnigy wattmeters for less but only one matched the WU meter's Wh and Ah values within a small margin, the other was way off. I tossed the second one in the trash. I tried an even cheaper blue wattmeter and it calculates the Ah/Wh about 8% low compared to my other two plus it gives instantaneous peak current values 25% above my other two wattmeters. And an inaccurate wattmeter isn't much good to anyone.

-R

Thanks Russell. Yes I think I've got the cheapest metal blue wattmeter (which gets oddly hot incidentally). I've changed the sharing settings on the image now so it's viewable, sorry. The peak current seems to correspond with the manufacturer claims for each of the controllers I've tried, but what you said about the Vrest and Vmin made sense - sag of 0.5v seems ridiculously low. So last night I splashed out and bought a CA3-DPS with shunt, because of the tiresome European speed limit restrictions, that should solve the wattmeter issues too.

My expenditure on ebike bits, to remedy the deficiencies of the bits I bought previously, reminds me a little of the song about the old lady who swallowed a fly. Path dependence I think economists call it.

 

[Russell]

I can't access the picture.

That difference between Vrest and Vmin is of course not correct. My logs indicate an internal resistance of 0.184 ohms for a voltage sag of 0.184V per Amp or just over 4.0V at 22A peak load. This relationship holds until almost 90% capacity has been used.

There are quite a few inline wattmeters available nowadays. I still consider the Watt's Up to be the best, though it is a bit pricey and not the most robust. I bought two Turnigy wattmeters for less but only one matched the WU meter's Wh and Ah values within a small margin, the other was way off. I tossed the second one in the trash. I tried an even cheaper blue wattmeter and it calculates the Ah/Wh about 8% low compared to my other two plus it gives instantaneous peak current values 25% above my other two wattmeters. And an inaccurate wattmeter isn't much good to anyone.

-R

Thanks Russell. Yes I think I've got the cheapest metal blue wattmeter (which gets oddly hot incidentally). I've changed the sharing settings on the image now so it's viewable, sorry. The peak current seems to correspond with the manufacturer claims for each of the controllers I've tried, but what you said about the Vrest and Vmin made sense - sag of 0.5v seems ridiculously low. So last night I splashed out and bought a CA3-DPS with shunt, because of the tiresome European speed limit restrictions, that should solve the wattmeter issues too.

My expenditure on ebike bits, to remedy the deficiencies of the bits I bought previously, reminds me a little of the song about the old lady who swallowed a fly. Path dependence I think economists call it.

I see the picture now. Yes that's the blue POS I was referring to, and yeah mine gets real hot too. With its inaccuracies it is of little use to me. here are my 3 wattmeters;



I use the Turnigy inside the rear bag on my Jeep bike with the battery since it withstands the heat the best. The battery is 12S LiPo 16Ah and I have yet to drain it so I just use the wattmeter for trip data and not for constant monitoring.

I hear you about expenses. I have a lot of E-bike parts but still no Cycle Analyst, it is on my "would be nice to have" list. :wink:

-R

 

[jonathan75]

...The other way is to use an accurate wattmeter so you can keep tabs on the battery and turn for home before it gets too low. I would even suggest purposely riding until the pack is fully drained one time so you can get the capacity data for it. That will give you confidence on subsequent rides.
-R

Really appreciate your thoughts on all this, very helpful. Do you know a good way of being sure I'm not draining my battery in a damaging way if I were to try this? Unfortunately my understanding of electronics just is really low and I might get things wrong. Perhaps you mean tripping the LVC or going to a certain floor voltage, through moderate but not, say, high current usage - and *then* charging asap in the nearest cafe etc. I've just read things about creating irreversible resistance inside the cells. What do they say about a little knowledge being a dangerous thing... that's me all over

 

[jonathan75]

It's not really practical unless you have controllers and displays that are automatic dual voltage. Even then, they can get a bit mixed up if you change fom 36v to 48v. It seems that they have some sort of memory so that they can deal with the difference between a disharged 48v battery (38v) and a charged 36v one (42v).

Thanks D8veh. Gareth from Kunteng emailed me thinking maybe the p5 parameters in the LCD3 could be adjusted in order to make the LCD3 recognise the correct battery kind. Do you think this would work? It would be with an S12S controller.

 

[Russell]

...The other way is to use an accurate wattmeter so you can keep tabs on the battery and turn for home before it gets too low. I would even suggest purposely riding until the pack is fully drained one time so you can get the capacity data for it. That will give you confidence on subsequent rides.
-R

Really appreciate your thoughts on all this, very helpful. Do you know a good way of being sure I'm not draining my battery in a damaging way if I were to try this? Unfortunately my understanding of electronics just is really low and I might get things wrong. Perhaps you mean tripping the LVC or going to a certain floor voltage, through moderate but not, say, high current usage - and *then* charging asap in the nearest cafe etc. I've just read things about creating irreversible resistance inside the cells. What do they say about a little knowledge being a dangerous thing... that's me all over

The purpose of the BMS is to protect the battery against overcharge/undercharge on a pack and cell level therefore you won't damage the pack if you drain it until the BMS trips. You don't want to drain the pack to cutoff on a regular basis since that reduces battery life but one time as a test of capacity won't hurt it. Keep an eye on the wattmeter because when the BMS trips the wattmeter will lose power. If the controller should happen to cut out first the wattmeter will still be active.

You can also just drain the pack at home. I use an extension cord that has Anderson connectors on one end and an ac receptacle on the other. I plug in a few lamps with incandescent bulbs as a load and drain the pack (slowly) that way. Again keep an eye on the wattmeter as the pack gets low.

-R