Simple wiring diagram for small craft

Discussion in 'OnBoard Electronics & Controls' started by CDK, Jun 8, 2009.

  1. BertKu
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    BertKu Senior Member

    I took one 6 Volt battery, a MOSFET as "diode" and a globe of 12 Volt/60 Watt. This I connected to a second MOSFET which acted as a "diode" and again a 12 Volt globe. I connected the globe to a 12 Volt battery.

    Guess what. ??? CDK You flipping right. Both globes burned as hell on 12 Volt , also the 6 Volt battery got a knock and moved up to over 8 Volt.
    Proving that the reverse voltage is not allowed to be greater than 1,3 Volt. The reason is that Vgs = > 0 Volt (12 Volt) At Vgs = 0 volt, the reverse voltage was > 6 Volt and the MOSFET acted as a "diode".

    That is flipping sad. I have to remove the replies , to avoid somebody having a mishaps. It still works on two batteries, provided the voltage difference is not more than 1,3 Volt. But that is crammy solution.

    Well one can only learn from errors.
    Thanks CDK
     
  2. BertKu
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    BertKu Senior Member

    I am still of the opinion that more than one Lead Acid battery parallel is a no go. Maybe when they are brand new, but certainly not when they are used and varies from each other.

    Back to the schottky diodes. I like to do a test and see what the real difference is, charging with or without schottky diodes.

    1) I will take a battery, charged it to 13,6 - 13,8 Volt
    2) load it with a load for 1/2 hour until battery is 1/2 discharged
    3) charge the battery without diode and take the time with a stopwatch how long it takes.
    4) load the same battery and discharge for exactly 1/2 hour again
    5) charge the battery with a schottky diode and see how much longer it takes to charge the same battery to the same 13,6 - 13,8 Volt

    But there are some other solutions.
    1) Find a voltage regulator, which is adjustable or could be modified from 14,4 Volt to 14,8 Volt to compensate for the schottky diode of 0.4 Volt
    2) Have the MOSFETS alternating if one has more than 2 batteries parallel by stepping mode the VGS from + to 0 and only have one charging a battery for a short period.
    3) The full voltage of different makes varies slightly per cell and select the lower voltage type per cell required to charge the cell fully. This depends on the chemical composition and metals used in the battery.
    i.e. if one select a cell with 2.3 Volt required x 6 = 13.8 Volt. or a cell which requires 2.4 Volt per cell x 6 = 14.4 Volt. If one uses Schottky diodes on the latter one, the battery will only be fully charged after a long time. If one uses the 13.8 Volt one, with schottky diodes the battery will be fully charged.
    (I phoned the battery manufacturer and put my questions to the lab)
    Bert
     
  3. BertKu
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    BertKu Senior Member

    Now I understand why my battery packed up when I drove 1200 km to Johannesburg. I have a diesel engine and if the battery only require 13.8 Volt,
    while being charged at 14.4 Volt, no wonder the battery got overcharged and packed up.
    Today , I am driving long distances with my bright lights on. Just to avoid that the battery gets overcharged. A gasoline/petrol engine allways uses some current, which avoid overcharge, but not a diesel engine.

    Thanks CDK for pointing this hiccup with a Mosfet out.
    Bert
     
  4. CDK
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    CDK retired engineer

    There is a protection diode on the MOSFET chip to prevent reverse biasing.
    For this particular one the border between life and death is given in the datasheet of Int'l Rectifier as 1.3 V. From my own experience I know that even a short negative spike turns a MOSFET into a normal FET with quite different parameters.
     
  5. BertKu
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    BertKu Senior Member

    Hold it, hold it, Hold it, you are right. It does mean then that it still could work with normal FET's. But where the hick do I get cheap 115 Ampere normal FET's ? Do they still make them?
    Bert

    p.s. what about 2 zenerdiodes in opposite direction, over such a normal FET, that should work then.
     
    Last edited: Mar 25, 2011
  6. BertKu
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    BertKu Senior Member

    You are clever CDK. It took me two IRFP064N to blow the reverse zenerdiode without blowing the chip itself also.
    Yes, it probably would work in that case.

    But I am not too sure to promote this route as the ideal solution.

    I must have been in wonderland, that I forgot about the reverse zenerdiode, which is a pure short circuit, if the voltage is reversed. I wonder why they do not place 2 zenerdiodes in opposite direction.

    Thanks CDK for the discussion. Great to discuss some topic, without being bashed by some rude person. Lucky he is no longer on the net.
    Bert
     
  7. BertKu
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    BertKu Senior Member

    Well, we all have sometimes egg on the face, this round, it is my turn. Sorry CDK, I misinterpreted your reply and blew 3 Mosfets, then to realize that even normal Fet’s has only a reverse voltage of 1,5 Volt. I always used Mosfets in resistive mode and with the Source to ground and Drain to the resistive side. Never had to bother about reverse polarity.

    Thus using Mosfets or Fets is indeed out of the question, like you said.

    However I am the opinion, rather to use Schottky diodes if batteries are used parallel with the consequence, that it will only be recharged to 95% and never fully charged. Then having a number of different make batteries or new and old batteries mixed, switched parallel. It is just an opinion which has worked for me. I believe and have proven to myself, that the batteries last longer.
    I did do the test to see how it actual performs.

    The same battery used for both test., just to make it accurate. Each having the same load for an half hour discharging the battery.

    Battery without Schottky diode-------------- Battery charging with Schottky diode
    Date----- Time---- Voltage ---------------------Date ------Time ------Voltage
    26/3 -----19h38---13.60 Volt -----------------28/3 ------15h25 ----13.60 Volt
    26/3 -----20h08 ---12.25 Volt---discharged----28/3 ------15h55-----12.17 Volt

    27/3------20h09 ---12.25 Volt—charging -----28/3 -------15h56-----12.20 Volt
    27/3------20h45---13.05 Volt-------------------28/3 -------16h30-----12.60 Volt
    27/3------21h17---13.10 Volt ------------------28/3 --------18h00----12.96 Volt
    27/3------22h40---13.20 Volt-------------------28/3---------20h20----13.05-Volt
    28/3------06h50---13.50 Volt-------------------28/3---------22h35----13.10 Volt
    28/3------10h40---13.59 Volt-------------------28/3 --------23h35----13.12 Volt
    28/3------1oh45---13.60 Volt------------------- 29/3--------06h55----13.21 Volt
    -------------------------------------------------29/3--------08h35----13.23 Volt
    In conclusion:
    1) It took 14hours 13 minutes to recharge the battery to the same level as it was original without diode
    2) It took 1 hour 8 minutes to charge to 13.10 Volt, which is about 95% without diode
    3) Versus, with diode , it took 7 hours 19 minutes also to charge to 13.10 Volt

    The test was done not with an alternator, but with an exponential charger. i.e. above 13 Volt it supplies lower charging current then an alternator.

    Why the discharged voltage was lower the second round , may have to do with ambient temperature
    Bert
     
  8. Joakim
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    Joakim Senior Member

    What are you trying to measure/prove? The battery voltage during charging is very hard to convert to actual state of charge. Does your charger regulate voltage? The type of the charger is very important factor on how a diode affects charging.
     
  9. BertKu
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    BertKu Senior Member

    Thank you Joakim,
    I trying to prove to myself that if you have a number of different make and age Lead Acid batteries parallel, it is better to use some schottky diodes.
    The first test I did was with a kind of smart Lead Acid Battery charger. Now I am testing various Car Alternators out and have already noticed that it varies from 14.25 Volt to 14,65 Volt regulating . The charging with an alternator is different then with a smart charger. I will do that test now also with a normal car alternator and voltage regulator. Just to see what bubbles.
    Too many people state "Don't use diodes" , while I am of the opinion that it is better practice.
    Bert
     
  10. BertKu
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    BertKu Senior Member

    Hi Joakim,

    I forgot to mention. When I wrote down the Voltage, I took the charging clamps off. Therefore the battery voltage does tell me something about the condition of the battery, nearly full or still a long way to go.
    bert
     
  11. DaveJ
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    DaveJ Senior Member

    @BertKu: the only real way to tell how charged or uncharged (condition) a lead acid battery is to detect how saturated the solution is. If the solution has no more lead to put back into the plates then it is fully charged, if the solution can't take any more lead then the battery is fully discharged (this is a simplistic point of view). You need an analyser in the solution to tell you this, otherwise your guessing just using the voltage level of the battery or the voltage your charging at (your plates could be saturated with lead and the solution can be at 80% saturated, so your voltage level will indicate high but the battery will not take long to be discharged). Different battery at different state of age will have different rates of acceptance and voltage required for that acceptance. Have you looked at diacs or triacs, a triac will block current in both directions until triggered.

    For everyone, if you want to see how bad having two or more static (by this i mean not being kept charge in one way or anyother) batteries in parallel without some isolation, stick a sensative current meter between the two posative terminals and over time watch it swing back and forth as the batteries discharge each other in an attempt to get to a equal state.
     
  12. BertKu
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    BertKu Senior Member

    Hi Dave,

    Thank you Dave, your last paragraph confirms the problem. Maybe a few batteries parallel of the same make and brand new, you may get away with murder, but not when they get older and one of them is being replaced.

    With regards to your first paragraph. That may well be the case with a liquid Lead Acid Battery, but not for gell batteries. Most modern batteries are today in solid form. Even my KIA diesel has no longer liquid, but gell.
    Thus the best way is to measure the voltage over the battery terminals, with the one side not connected and a few minutes after the charger is disconnected. I guarantee you that if the meter shows 13,4 Volt, your battery is pritty full.

    The problem with Triacs and silicon diodes, the Forward Voltage drop is too high to my likings, between 0.5 and up to 1,5 volt, while Schottky is between 0,3 and 0,45 Volt. Depending on temperature and current.

    bert
     
  13. CDK
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    CDK retired engineer

    That was once true but nowadays very impractical. Without drilling a hole you can get a pretty good indication using a high current shunt for a few seconds and watching the voltage drop. Smart chargers gather their display information that way.

    Not a very smart remark. Diacs in a 12V system are as useful as a piece of rope: they do not conduct below 28V. A triac doesn't revert to a non-conductive state in a DC circuit.
     
  14. BertKu
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    BertKu Senior Member

    Dave, it is now your turn to have egg on your face. Don't worry I had my turn. We all do have at one stage of another. CDK is a sharp engineer and I appreciate his straightforward remarks. Have a good grin.
    Bert
     

  15. Joakim
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    Joakim Senior Member

    That doesn't really help much, which can easily be seen from your measured voltages. A full lead acid battery is only about 12.7 depending on type, condition and temperature, never over 13 V. It takes hours after charging for the voltage to stabilize unless some load is used.
     
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