walkin' home alone

DIY Builds Battery Builds
1

So, the other day, for the first time, I completely ran out of battery, and I was miles from my car.

So obviously I do not have the proper discharge chart in terms of real-world use. I was on a completely flat trail and I started my ride at 50% battery. At 30% I turned around. At about 20% it pooped out. So the discharge rate in terms of voltage drop at the bottom of the battery capacity is far greater than at the top. The below is the table I use, which I got from (somewhere) and probably represents a much lower current draw.

My question is does anyone have a field tested and known accurate table?

Otherwise I’m going to consider the old 20% (38.8v) to be the new 0% and re-math the below table.

bat% batV
100 50.4
99 50.22
98 50.12
97 49.94
96 49.85
95 49.66
94 49.57
93 49.38
92 49.29
91 49.11
90 48.92
89 48.83
88 48.65
87 48.55
86 48.37
85 48.28
84 48.09
83 47.91
82 47.82
81 47.63
80 47.54
79 47.35
78 47.26
77 47.08
76 46.98
75 46.8
74 46.62
73 46.52
72 46.34
71 46.25
70 46.06
69 45.97
68 45.78
67 45.69
66 45.51
65 45.32
64 45.23
63 45.05
62 44.95
61 44.77
60 44.68
59 44.49
58 44.31
57 44.22
56 44.03
55 43.94
54 43.75
53 43.66
52 43.48
51 43.38
50 43.2
49 43.02
48 42.92
47 42.74
46 42.65
45 42.46
44 42.37
43 42.18
42 42.09
41 41.91
40 41.72
39 41.63
38 41.45
37 41.35
36 41.17
35 41.08
34 40.89
33 40.71
32 40.62
31 40.43
30 40.34
29 40.15
28 40.06
27 39.88
26 39.78
25 39.6
24 39.42
23 39.32
22 39.14
21 39.05
20 38.86
19 38.77
18 38.58
17 38.49
16 38.31
15 38.12
14 38.03
13 37.85
12 37.75
11 37.57
10 37.48
9 37.29
8 37.11
7 37.02
6 36.83
5 36.74
4 36.55
3 36.46
2 36.28
1 36.18
0 36

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2

I hope your battery survives. Most batteries that have died on me, have died in similar circumstances.

Do you know if it’s balanced?

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3

I think you’re chart is good but it’s too linear. @b264 might be on to something if your pack is starting to age or get out of balance you wont be able to reliably use those lower voltages and the 0-20% wont have much usable capacity. If your pack has any sagging it will drop to the cutoff voltage trying to draw much amperage and youll start the walk of shame.

Can you monitor the pack at a cell level with a smart bms? Can you see a live reading when under load? Those might be a better place to add a little spice to your chart and allow for real world conditions.

I try to get back with 15-20% left on my pack so I don’t have to let up on the throttle on the way home. Ymmv

4 Likes
4

You are definitely right, I need to look into balance. I have it on an off the shelf BMS which I foolishly assume is doing all the right things. I used to have off-board balance charging which was spot on every time, but took forever.

The pack is quite new, but I’ll hook that bluetooth dongle back up to it and see what the cell voltages are looking like.

I was caught off guard on the whole situation because it’s not my daily driver, but it’s almost exactly the same setup - same number/battery capacity, and the daily is a 40 mile board (theoretically, but I’ve tested it to 30 miles). How this one could basically give up the ghost at 20 miles is a mystery to me. (two rides, the first 10 miles got me from 94% to 50%)

5

Aren’t these assumptions based upon accepting the stock cutoffs in vesc tool? I set mine to cut off somewhat sooner than the suggested cutoff.

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