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Battery Monitor / Motor Monitor — Parallax Forums

Battery Monitor / Motor Monitor

bdickensbdickens Posts: 110
edited 2010-04-23 15:00 in Propeller 1
This may be a common question, but I can't seem to find a good source for info. So if someone has it, aim me at it and i'll go digging (or my son will)

For his summer project, my son has decided to replace my boats electronics with an integrated solution. Sounds easy enough, but it's a bit more complex. The boat is electric powered (Etek) at 48V (4 - 12v batteries). In addition to the simple stuff (water speed, ground speed, and temperature) he would like to add a sensor for battery charge (state of all 4 batteries individually and together) and motor draw. I started to look for threads and drew a blank in propeller. But I figure some of you may know better sites and simple circuits that could be interfaced to his prop chip.

Thanks

Comments

  • RaymanRayman Posts: 14,887
    edited 2010-04-21 20:07
    Well, I'm using a MAX1270 and a resistive voltage divider to measure the voltage across four 12V batteries in series.

    If you're charging and discharging the batteries in series, there's not much point to measuring them individually.
    But if you really need to, I suppose one easy way to do that is to measure from ground to the positive terminal of each battery and then doing some math to figure out each battery's voltage.

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  • bdickensbdickens Posts: 110
    edited 2010-04-21 20:42
    They all have their own charging circuit. Two marine dual chargers (and I am not letting him mess with that). So each one gets charged independently. But they do discharge in series (obviously). I think the reason for watching individual batteries is to make sure they all perform and stay healthy.

    Thanks for the suggestions. I'll start there.
  • RaymanRayman Posts: 14,887
    edited 2010-04-21 21:26
    A·more fun way is to use two Prop pins and a few passive parts to do sigma-delta ADC:

    Look at this app:

    http://www.pulsedpower.net/Applets/Electronics/SigmaDeltaADC/SigmaDeltab.html

    if you set:· R1=4k, R2=100k, R3=Not used, R4=5k

    then the range is -6 to 75 volts, which should be pretty good...

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  • mctriviamctrivia Posts: 3,772
    edited 2010-04-21 21:46
    if building your owns Sigma delta ADC you need to place the components close to the prop chip(<1").

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  • JonathanJonathan Posts: 1,023
    edited 2010-04-22 15:32
    Battery voltage is not a reliable indicator of state of charge, unless you have allowed the battery to rest for 24 hours (no charge in or out). You could track amp hours in and out (which is what I do with my off grid AE system). Probably not worth it if the boat is a power vessel (motor when moving, charging batteries) but would be useful on a sailboat to know when to fire up and charge.

    Funny, I'm working on my Prop based charging controller for mys system today.

    Jonathan

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  • RaymanRayman Posts: 14,887
    edited 2010-04-22 15:44
    I've found that the battery voltage for lead-acid batteries is a very good indicator of charge state...

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  • LeonLeon Posts: 7,620
    edited 2010-04-22 15:54
    How about adding a hygrometer to each battery and measuring the float position optically? smile.gif

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  • JonathanJonathan Posts: 1,023
    edited 2010-04-22 16:16
    Ray,

    Sorry to disagree, but voltage isn't a really good indicator of SOC (state of charge). This isn't just my opinion, it is widely held by battery makers and users. I can say from my personal experiences of liviing off of batteries that it is true. (10 years living aboard and a year living off grid). Off gridders usually track amp hours for a rough idea, then check SG (specific gravity). Battery temp, charges in and out all contribute to the inaccuarcy of voltage.

    Leon,

    You'd have to have one for each cell to really monitor the overall health, as there can be quite a difference between cells, especailly if they haven't been equalized for a while. Although in a marine setting this is usually less of a factor, as the motiuon of the boat prevents striation of the electrolyte.

    Jonathan

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  • RaymanRayman Posts: 14,887
    edited 2010-04-22 16:49
    Jonathan: Well I suppose it depends on several things, but to first order the voltage is a good indication of charge for a lead-acid battery. Perhaps with my setup, new, sealed batteries in a temperature controlled setting, I don't see the same affects you have.

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  • RaymanRayman Posts: 14,887
    edited 2010-04-22 16:50
    Question: Cars often have battery voltmeters on the dashboard... Do boats have the same thing?

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  • JonathanJonathan Posts: 1,023
    edited 2010-04-22 17:13
    Ray,

    for an example, I often see a night time voltage on my battery bank (12V, 1200aH, 10 * 6V batteries in series/parallel) of 12.3V or even lower, in spite of the fact that I have used less than 100aH out of the bank since it was charged. If you look at a voltage SOC chart, if my batteries *really* were that low (~65% charge), I'd freak. Think of the Star Trek red alert noise going off in my head. My batteries cost me almost a grand, so I baby them and never take out more than 20%.

    Boats usually do have a voltmeter. There are a bunch of monitoring systems out there. I have a marine monitor here at the house that I have been using while I build my own, better monitor. It tracks amp hours in and out. It does pretty well at giving you a good idea of state of charge, but over time the amp hour tracking gets a little off. What I do is charge my bank to about 90% daily for a few days, then charge fully. As I don't have enough solar to run my place, I have to run a generator, so this scheme is a decent compromise between fuel consumption and battery care.

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  • JackBakJackBak Posts: 45
    edited 2010-04-22 17:29
    Ok Jonathan, tracking aHrs in and out is quite elaborate. I am in the process of doing something like what Ray is talking
    about for some RC LiPo batteries. Maybe your method is specific to Lead-Acid because everything I have read
    about LiPos suggests just watching the Voltage curve, both for charging and discharging.

    Is this Lead-Acid only data we are talking about here?

    -Jack
  • JonathanJonathan Posts: 1,023
    edited 2010-04-22 17:57
    Yes, I am talking about flooded lead acid batteries. Different types do indeed charge differently. And yeah, tracking aH is more complex. But then, there is no AAA for a jump start at sea. :-0 And for me, I can't afford to replace my batteries and want to get the max service life.

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  • bdickensbdickens Posts: 110
    edited 2010-04-22 18:24
    Lots of comments.

    Leon - I'm hoping that isn't needed, but candidly, I have been looking for a way to measure salinity so if you know a clever, waterproof way to measure the hydrometer position, I am there.

    The batteries are deep cycle, lead acid. They ONLY charge at the dock and through the small solar trickle charge. But they are also the sole source of power for the boat. We get about 7hp at the prop. It's a modified outboard ala the Glen-L designs.

    I could probably buy something, but that defeats his project. He wants to learn, I just don't want to paddle if he scews up. Tracking amp/hrs would be nice. I'd be modestly happy if I could have enough data to predict my available amp hours. ( to someone's post, No jump starts in the ocean). Also an indication of how long I can run at what speeds.

    Keep the thoughts coming
  • RaymanRayman Posts: 14,887
    edited 2010-04-22 18:28
    Well, I'd add a digital thermometer at the batteries. I just took a quick look and there is definitely a temperature affect on lead-acid batteries. So, knowing the battery temperature is probably nice too...

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  • JonathanJonathan Posts: 1,023
    edited 2010-04-22 18:49
    Ray is right, it needs to be temp. compensated to be accurate. Take a look at the Allegro ACS758 and similar for current sensors. I use the RTC object for a time base. I have a GPS that I use to get the correct time once per day and reset the RTC object to keep it on track. I'm guessing you have a GPS onboard, so you may be able to use the same scheme.

    Jonathan

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  • RvnPhnxRvnPhnx Posts: 36
    edited 2010-04-22 23:05
    Thought about using one of these (or something similar)?
    www.sparkfun.com/commerce/product_info.php?products_id=9028

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  • bdickensbdickens Posts: 110
    edited 2010-04-23 13:52
    RvnPhnx - Darn. I knew about Atto and I totally missed that one. Great find. Thanks.

    Jonathan / Rayman - Will do. We have more than enough temperature sensors !

    GPS synchronizing RTC. Another thought that had not occurred to me but it's on the whiteboard. That may be just as useful on my summer project as opposed to his. Another excellent thought. (we both tackle projects starting on June 1 with the goal of completing by August 1).

    Thank you all. Keep the thoughts coming
  • JonathanJonathan Posts: 1,023
    edited 2010-04-23 15:00
    bdickens,

    If you locate the temp. sensor on the battery case, make sure it is well insulated on the side away from the battery. You can also strap it to a battery terminal. I'm actually working on a glass tube that sits in the electolyte. Otherwise enviromental factors can set your temp. comp. way off.

    If you do go the amp hour route, remember that battery charging is only 70- 80% effecient. So, if your amp hour meter reads that you have used 100aH, you will need to replace between 120-130 aH to recharge. During your first few runs you need to verify the SG with a hydrometer, the adjust your effeciency compennsation accordingly. Get a temp. compensated hydrometer like the EX Red SP101. Cheap and works great, no need to manually compensate for temp.

    Jonathan

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