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Charging AGM batteries from car alternator.

Discussion in 'Mobile Radio Systems' started by K7POC, Nov 21, 2014.

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  1. K7POC

    K7POC Ham Member QRZ Page

    I just bought a werker dc12-80p 80ah AGM battery. Going to stick it in my jeep to power my hf/vhf stuff when parked. I was just going to install a West mountain ISOpwr and be done with it. However, I'm concerned that I might fry the battery connecting it directly to the alt. Any thoughts/suggestions??

  2. K0BG

    K0BG Ham Member QRZ Page

    All lead acid, SLI batteries can be charged using the existing vehicle charging systems—with a caveat or two.

    AGMs, like the Optima series, have slightly higher resting voltage. For example, a nominal SLI will settle down to about 12.2 volts. An Optima rests at about 12.4. Eventually, when the two are connected together, the Optima will settle at the same voltage as a flooded lead acid. Doesn't hurt anything, but you need to be aware of it.

    If you're going to use the battery as standalone, you will need a battery booster to maintain the voltage at about 13.8 to the transceiver. If you don't, there will be an increase in the IMD level, and a decrease in output power. In a mobile, they should be paralleled, with only fuses as a separator. You might read my web site about this issue in the Alternator article.
  3. KD0CAC

    KD0CAC Ham Member QRZ Page

    I would do 2 things , 1st is a battery isolator - this allows you to run the AGM / or any 2nd battery with out draining the starter battery and give a separate charge circuit from alternator [ the regulator in the alternator may see the starting battery as fully charged and may not put charge current to 2nd battery ]
    Then charging the secondary can be problematic - for long battery life having a charge controller that matches the charging profile of the 2nd battery will give longer life , in addition to that , is that you are in charge of turning the charge source off [ engine running ] with out some kind of monitoring from charge controller , you may not run the engine long enough or at a high enough RPM - at idle there is little or no charging from alt. , and lead acid batteries , as part of there charge profile , should be at a relatively high current rate , AMG less so than FLA .
  4. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    The MFJ Battery booster seems to work pretty good, but its PRICEY! $150! I used my oldW4RRY battery booster to boost the AUTO voltage to recharge the AGMs (I have 3 to run the radio) A switch selects one of the 3 and the other 2 get recharged by the 2nd booster. Seems to work OK now.
  5. NH7RO

    NH7RO Ham Member QRZ Page

    I've often seen a battery booster for $59.95 in a QRZ ad banner that is run by "Grumpyshop" in Texas; don't know much about it but at that price it'd be worth checking out. I might get one for my mobile setup soon and if I do I'll mention or review it here at some point.

    At any rate a battery isolator is a very good idea and a battery (voltage) booster is another.

    73, Jeff
  6. K0BG

    K0BG Ham Member QRZ Page

    Allow me to comment further on a battery isolator.

    Regular old battery isolators only use diodes, and the forward drop can be excessive (>1 volt). Rather buy one which uses FET switches, as the drop is (<.2 volts). Hellroaring is one such make.

    All this said, if you have a late-model vehicle which uses a Battery Monitoring System, you run the risk of turning on the Maintenance Indicator Lamp (Check Engine, etc.) when the auxiliary battery comes on-line.
  7. K7POC

    K7POC Ham Member QRZ Page

    It's a 99 jeep cherokee so not new enough to mess things up. I bought 10awg wire to run. However I was thinking I should upgrade to 6awg.
  8. K0BG

    K0BG Ham Member QRZ Page

    Either that or size 4! And the difference in cost isn't that much.

    I have two batteries in my Ridgeline, because I run high power (≈550 watts PEP output). The one in the trunk is fed from the front battery with #2. Even then, the peak voltage drop is close to .5 volts, which is the rule of thumb maximum. Both batteries are 55 Ah Optima RedTops.
  9. K8ERV

    K8ERV Ham Member QRZ Page

    Rather than one very thick wire it may be more practical to run two or more smaller wires in parallel.

    TOM K8ERV Montrose Colo
  10. K0BG

    K0BG Ham Member QRZ Page

    Tom, I agree with you. Perhaps I should use the word "equivalent" instead. However, if there was a large-scale discussion about this issue, the purists would have a field day!

    Then there is the other issue, which assumes that two #6s, will carry the same current as one #4. Doubling the capacity of two #6 in air (not always the case), exceeds the capacity of one #4 by nearly 15 amps! The same goes for the European standards. The Japanese are the smart bunch in this case, as their wire size is based on a linear scale.

    Where all of this started, I haven't a clue. It is, however, an interesting study of secular and /or regional professional training. If nothing else, you and I could have a field day debating the issue. After all, there is something to be said about peer reckoning being a good thing!
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